sequencer: allow disabling conflict advice

[alt-git.git] / builtin / merge.c

/*
 * Builtin "git merge"
 *
 * Copyright (c) 2008 Miklos Vajna <vmiklos@frugalware.org>
 *
 * Based on git-merge.sh by Junio C Hamano.
 */

#define USE_THE_INDEX_VARIABLE
#include "builtin.h"
#include "abspath.h"
#include "advice.h"
#include "config.h"
#include "editor.h"
#include "environment.h"
#include "gettext.h"
#include "hex.h"
#include "object-name.h"
#include "parse-options.h"
#include "lockfile.h"
#include "run-command.h"
#include "hook.h"
#include "diff.h"
#include "diff-merges.h"
#include "refs.h"
#include "refspec.h"
#include "commit.h"
#include "diffcore.h"
#include "path.h"
#include "revision.h"
#include "unpack-trees.h"
#include "cache-tree.h"
#include "dir.h"
#include "color.h"
#include "rerere.h"
#include "help.h"
#include "merge.h"
#include "merge-recursive.h"
#include "merge-ort-wrappers.h"
#include "resolve-undo.h"
#include "remote.h"
#include "fmt-merge-msg.h"
#include "sequencer.h"
#include "string-list.h"
#include "tag.h"
#include "alias.h"
#include "branch.h"
#include "commit-reach.h"
#include "wt-status.h"
#include "commit-graph.h"

#define DEFAULT_TWOHEAD (1<<0)
#define DEFAULT_OCTOPUS (1<<1)
#define NO_FAST_FORWARD (1<<2)
#define NO_TRIVIAL      (1<<3)

struct strategy {
        const char *name;
        unsigned attr;
};

static const char * const builtin_merge_usage[] = {
        N_("git merge [<options>] [<commit>...]"),
        "git merge --abort",
        "git merge --continue",
        NULL
};

static int show_diffstat = 1, shortlog_len = -1, squash;
static int option_commit = -1;
static int option_edit = -1;
static int allow_trivial = 1, have_message, verify_signatures;
static int check_trust_level = 1;
static int overwrite_ignore = 1;
static struct strbuf merge_msg = STRBUF_INIT;
static struct strategy **use_strategies;
static size_t use_strategies_nr, use_strategies_alloc;
static struct strvec xopts = STRVEC_INIT;
static const char *branch;
static char *branch_mergeoptions;
static int verbosity;
static int allow_rerere_auto;
static int abort_current_merge;
static int quit_current_merge;
static int continue_current_merge;
static int allow_unrelated_histories;
static int show_progress = -1;
static int default_to_upstream = 1;
static int signoff;
static const char *sign_commit;
static int autostash;
static int no_verify;
static char *into_name;

static struct strategy all_strategy[] = {
        { "recursive",  NO_TRIVIAL },
        { "octopus",    DEFAULT_OCTOPUS },
        { "ort",        DEFAULT_TWOHEAD | NO_TRIVIAL },
        { "resolve",    0 },
        { "ours",       NO_FAST_FORWARD | NO_TRIVIAL },
        { "subtree",    NO_FAST_FORWARD | NO_TRIVIAL },
};

static const char *pull_twohead, *pull_octopus;

enum ff_type {
        FF_NO,
        FF_ALLOW,
        FF_ONLY
};

static enum ff_type fast_forward = FF_ALLOW;

static const char *cleanup_arg;
static enum commit_msg_cleanup_mode cleanup_mode;

static int option_parse_message(const struct option *opt,
                                const char *arg, int unset)
{
        struct strbuf *buf = opt->value;

        if (unset)
                strbuf_setlen(buf, 0);
        else if (arg) {
                strbuf_addf(buf, "%s%s", buf->len ? "\n\n" : "", arg);
                have_message = 1;
        } else
                return error(_("switch `m' requires a value"));
        return 0;
}

static enum parse_opt_result option_read_message(struct parse_opt_ctx_t *ctx,
                                                 const struct option *opt,
                                                 const char *arg_not_used,
                                                 int unset)
{
        struct strbuf *buf = opt->value;
        const char *arg;

        BUG_ON_OPT_ARG(arg_not_used);
        if (unset)
                BUG("-F cannot be negated");

        if (ctx->opt) {
                arg = ctx->opt;
                ctx->opt = NULL;
        } else if (ctx->argc > 1) {
                ctx->argc--;
                arg = *++ctx->argv;
        } else
                return error(_("option `%s' requires a value"), opt->long_name);

        if (buf->len)
                strbuf_addch(buf, '\n');
        if (ctx->prefix && !is_absolute_path(arg))
                arg = prefix_filename(ctx->prefix, arg);
        if (strbuf_read_file(buf, arg, 0) < 0)
                return error(_("could not read file '%s'"), arg);
        have_message = 1;

        return 0;
}

static struct strategy *get_strategy(const char *name)
{
        int i;
        struct strategy *ret;
        static struct cmdnames main_cmds, other_cmds;
        static int loaded;
        char *default_strategy = getenv("GIT_TEST_MERGE_ALGORITHM");

        if (!name)
                return NULL;

        if (default_strategy &&
            !strcmp(default_strategy, "ort") &&
            !strcmp(name, "recursive")) {
                name = "ort";
        }

        for (i = 0; i < ARRAY_SIZE(all_strategy); i++)
                if (!strcmp(name, all_strategy[i].name))
                        return &all_strategy[i];

        if (!loaded) {
                struct cmdnames not_strategies;
                loaded = 1;

                memset(&not_strategies, 0, sizeof(struct cmdnames));
                load_command_list("git-merge-", &main_cmds, &other_cmds);
                for (i = 0; i < main_cmds.cnt; i++) {
                        int j, found = 0;
                        struct cmdname *ent = main_cmds.names[i];
                        for (j = 0; !found && j < ARRAY_SIZE(all_strategy); j++)
                                if (!xstrncmpz(all_strategy[j].name, ent->name, ent->len))
                                        found = 1;
                        if (!found)
                                add_cmdname(&not_strategies, ent->name, ent->len);
                }
                exclude_cmds(&main_cmds, &not_strategies);
        }
        if (!is_in_cmdlist(&main_cmds, name) && !is_in_cmdlist(&other_cmds, name)) {
                fprintf(stderr, _("Could not find merge strategy '%s'.\n"), name);
                fprintf(stderr, _("Available strategies are:"));
                for (i = 0; i < main_cmds.cnt; i++)
                        fprintf(stderr, " %s", main_cmds.names[i]->name);
                fprintf(stderr, ".\n");
                if (other_cmds.cnt) {
                        fprintf(stderr, _("Available custom strategies are:"));
                        for (i = 0; i < other_cmds.cnt; i++)
                                fprintf(stderr, " %s", other_cmds.names[i]->name);
                        fprintf(stderr, ".\n");
                }
                exit(1);
        }

        CALLOC_ARRAY(ret, 1);
        ret->name = xstrdup(name);
        ret->attr = NO_TRIVIAL;
        return ret;
}

static void append_strategy(struct strategy *s)
{
        ALLOC_GROW(use_strategies, use_strategies_nr + 1, use_strategies_alloc);
        use_strategies[use_strategies_nr++] = s;
}

static int option_parse_strategy(const struct option *opt UNUSED,
                                 const char *name, int unset)
{
        if (unset)
                return 0;

        append_strategy(get_strategy(name));
        return 0;
}

static struct option builtin_merge_options[] = {
        OPT_SET_INT('n', NULL, &show_diffstat,
                N_("do not show a diffstat at the end of the merge"), 0),
        OPT_BOOL(0, "stat", &show_diffstat,
                N_("show a diffstat at the end of the merge")),
        OPT_BOOL(0, "summary", &show_diffstat, N_("(synonym to --stat)")),
        { OPTION_INTEGER, 0, "log", &shortlog_len, N_("n"),
          N_("add (at most <n>) entries from shortlog to merge commit message"),
          PARSE_OPT_OPTARG, NULL, DEFAULT_MERGE_LOG_LEN },
        OPT_BOOL(0, "squash", &squash,
                N_("create a single commit instead of doing a merge")),
        OPT_BOOL(0, "commit", &option_commit,
                N_("perform a commit if the merge succeeds (default)")),
        OPT_BOOL('e', "edit", &option_edit,
                N_("edit message before committing")),
        OPT_CLEANUP(&cleanup_arg),
        OPT_SET_INT(0, "ff", &fast_forward, N_("allow fast-forward (default)"), FF_ALLOW),
        OPT_SET_INT_F(0, "ff-only", &fast_forward,
                      N_("abort if fast-forward is not possible"),
                      FF_ONLY, PARSE_OPT_NONEG),
        OPT_RERERE_AUTOUPDATE(&allow_rerere_auto),
        OPT_BOOL(0, "verify-signatures", &verify_signatures,
                N_("verify that the named commit has a valid GPG signature")),
        OPT_CALLBACK('s', "strategy", NULL, N_("strategy"),
                N_("merge strategy to use"), option_parse_strategy),
        OPT_STRVEC('X', "strategy-option", &xopts, N_("option=value"),
                N_("option for selected merge strategy")),
        OPT_CALLBACK('m', "message", &merge_msg, N_("message"),
                N_("merge commit message (for a non-fast-forward merge)"),
                option_parse_message),
        { OPTION_LOWLEVEL_CALLBACK, 'F', "file", &merge_msg, N_("path"),
                N_("read message from file"), PARSE_OPT_NONEG,
                NULL, 0, option_read_message },
        OPT_STRING(0, "into-name", &into_name, N_("name"),
                   N_("use <name> instead of the real target")),
        OPT__VERBOSITY(&verbosity),
        OPT_BOOL(0, "abort", &abort_current_merge,
                N_("abort the current in-progress merge")),
        OPT_BOOL(0, "quit", &quit_current_merge,
                N_("--abort but leave index and working tree alone")),
        OPT_BOOL(0, "continue", &continue_current_merge,
                N_("continue the current in-progress merge")),
        OPT_BOOL(0, "allow-unrelated-histories", &allow_unrelated_histories,
                 N_("allow merging unrelated histories")),
        OPT_SET_INT(0, "progress", &show_progress, N_("force progress reporting"), 1),
        { OPTION_STRING, 'S', "gpg-sign", &sign_commit, N_("key-id"),
          N_("GPG sign commit"), PARSE_OPT_OPTARG, NULL, (intptr_t) "" },
        OPT_AUTOSTASH(&autostash),
        OPT_BOOL(0, "overwrite-ignore", &overwrite_ignore, N_("update ignored files (default)")),
        OPT_BOOL(0, "signoff", &signoff, N_("add a Signed-off-by trailer")),
        OPT_BOOL(0, "no-verify", &no_verify, N_("bypass pre-merge-commit and commit-msg hooks")),
        OPT_END()
};

static int save_state(struct object_id *stash)
{
        int len;
        struct child_process cp = CHILD_PROCESS_INIT;
        struct strbuf buffer = STRBUF_INIT;
        struct lock_file lock_file = LOCK_INIT;
        int fd;
        int rc = -1;

        fd = repo_hold_locked_index(the_repository, &lock_file, 0);
        refresh_index(&the_index, REFRESH_QUIET, NULL, NULL, NULL);
        if (0 <= fd)
                repo_update_index_if_able(the_repository, &lock_file);
        rollback_lock_file(&lock_file);

        strvec_pushl(&cp.args, "stash", "create", NULL);
        cp.out = -1;
        cp.git_cmd = 1;

        if (start_command(&cp))
                die(_("could not run stash."));
        len = strbuf_read(&buffer, cp.out, 1024);
        close(cp.out);

        if (finish_command(&cp) || len < 0)
                die(_("stash failed"));
        else if (!len)          /* no changes */
                goto out;
        strbuf_setlen(&buffer, buffer.len-1);
        if (repo_get_oid(the_repository, buffer.buf, stash))
                die(_("not a valid object: %s"), buffer.buf);
        rc = 0;
out:
        strbuf_release(&buffer);
        return rc;
}

static void read_empty(const struct object_id *oid)
{
        struct child_process cmd = CHILD_PROCESS_INIT;

        strvec_pushl(&cmd.args, "read-tree", "-m", "-u", empty_tree_oid_hex(),
                     oid_to_hex(oid), NULL);
        cmd.git_cmd = 1;

        if (run_command(&cmd))
                die(_("read-tree failed"));
}

static void reset_hard(const struct object_id *oid)
{
        struct child_process cmd = CHILD_PROCESS_INIT;

        strvec_pushl(&cmd.args, "read-tree", "-v", "--reset", "-u",
                     oid_to_hex(oid), NULL);
        cmd.git_cmd = 1;

        if (run_command(&cmd))
                die(_("read-tree failed"));
}

static void restore_state(const struct object_id *head,
                          const struct object_id *stash)
{
        struct child_process cmd = CHILD_PROCESS_INIT;

        reset_hard(head);

        if (is_null_oid(stash))
                goto refresh_cache;

        strvec_pushl(&cmd.args, "stash", "apply", "--index", "--quiet", NULL);
        strvec_push(&cmd.args, oid_to_hex(stash));

        /*
         * It is OK to ignore error here, for example when there was
         * nothing to restore.
         */
        cmd.git_cmd = 1;
        run_command(&cmd);

refresh_cache:
        discard_index(&the_index);
        if (repo_read_index(the_repository) < 0)
                die(_("could not read index"));
}

/* This is called when no merge was necessary. */
static void finish_up_to_date(void)
{
        if (verbosity >= 0) {
                if (squash)
                        puts(_("Already up to date. (nothing to squash)"));
                else
                        puts(_("Already up to date."));
        }
        remove_merge_branch_state(the_repository);
}

static void squash_message(struct commit *commit, struct commit_list *remoteheads)
{
        struct rev_info rev;
        struct strbuf out = STRBUF_INIT;
        struct commit_list *j;
        struct pretty_print_context ctx = {0};

        printf(_("Squash commit -- not updating HEAD\n"));

        repo_init_revisions(the_repository, &rev, NULL);
        diff_merges_suppress(&rev);
        rev.commit_format = CMIT_FMT_MEDIUM;

        commit->object.flags |= UNINTERESTING;
        add_pending_object(&rev, &commit->object, NULL);

        for (j = remoteheads; j; j = j->next)
                add_pending_object(&rev, &j->item->object, NULL);

        setup_revisions(0, NULL, &rev, NULL);
        if (prepare_revision_walk(&rev))
                die(_("revision walk setup failed"));

        ctx.abbrev = rev.abbrev;
        ctx.date_mode = rev.date_mode;
        ctx.fmt = rev.commit_format;

        strbuf_addstr(&out, "Squashed commit of the following:\n");
        while ((commit = get_revision(&rev)) != NULL) {
                strbuf_addch(&out, '\n');
                strbuf_addf(&out, "commit %s\n",
                        oid_to_hex(&commit->object.oid));
                pretty_print_commit(&ctx, commit, &out);
        }
        write_file_buf(git_path_squash_msg(the_repository), out.buf, out.len);
        strbuf_release(&out);
        release_revisions(&rev);
}

static void finish(struct commit *head_commit,
                   struct commit_list *remoteheads,
                   const struct object_id *new_head, const char *msg)
{
        struct strbuf reflog_message = STRBUF_INIT;
        const struct object_id *head = &head_commit->object.oid;

        if (!msg)
                strbuf_addstr(&reflog_message, getenv("GIT_REFLOG_ACTION"));
        else {
                if (verbosity >= 0)
                        printf("%s\n", msg);
                strbuf_addf(&reflog_message, "%s: %s",
                        getenv("GIT_REFLOG_ACTION"), msg);
        }
        if (squash) {
                squash_message(head_commit, remoteheads);
        } else {
                if (verbosity >= 0 && !merge_msg.len)
                        printf(_("No merge message -- not updating HEAD\n"));
                else {
                        update_ref(reflog_message.buf, "HEAD", new_head, head,
                                   0, UPDATE_REFS_DIE_ON_ERR);
                        /*
                         * We ignore errors in 'gc --auto', since the
                         * user should see them.
                         */
                        run_auto_maintenance(verbosity < 0);
                }
        }
        if (new_head && show_diffstat) {
                struct diff_options opts;
                repo_diff_setup(the_repository, &opts);
                init_diffstat_widths(&opts);
                opts.output_format |=
                        DIFF_FORMAT_SUMMARY | DIFF_FORMAT_DIFFSTAT;
                opts.detect_rename = DIFF_DETECT_RENAME;
                diff_setup_done(&opts);
                diff_tree_oid(head, new_head, "", &opts);
                diffcore_std(&opts);
                diff_flush(&opts);
        }

        /* Run a post-merge hook */
        run_hooks_l("post-merge", squash ? "1" : "0", NULL);

        if (new_head)
                apply_autostash_ref(the_repository, "MERGE_AUTOSTASH");
        strbuf_release(&reflog_message);
}

/* Get the name for the merge commit's message. */
static void merge_name(const char *remote, struct strbuf *msg)
{
        struct commit *remote_head;
        struct object_id branch_head;
        struct strbuf bname = STRBUF_INIT;
        struct merge_remote_desc *desc;
        const char *ptr;
        char *found_ref = NULL;
        int len, early;

        strbuf_branchname(&bname, remote, 0);
        remote = bname.buf;

        oidclr(&branch_head);
        remote_head = get_merge_parent(remote);
        if (!remote_head)
                die(_("'%s' does not point to a commit"), remote);

        if (repo_dwim_ref(the_repository, remote, strlen(remote), &branch_head,
                          &found_ref, 0) > 0) {
                if (starts_with(found_ref, "refs/heads/")) {
                        strbuf_addf(msg, "%s\t\tbranch '%s' of .\n",
                                    oid_to_hex(&branch_head), remote);
                        goto cleanup;
                }
                if (starts_with(found_ref, "refs/tags/")) {
                        strbuf_addf(msg, "%s\t\ttag '%s' of .\n",
                                    oid_to_hex(&branch_head), remote);
                        goto cleanup;
                }
                if (starts_with(found_ref, "refs/remotes/")) {
                        strbuf_addf(msg, "%s\t\tremote-tracking branch '%s' of .\n",
                                    oid_to_hex(&branch_head), remote);
                        goto cleanup;
                }
        }

        /* See if remote matches <name>^^^.. or <name>~<number> */
        for (len = 0, ptr = remote + strlen(remote);
             remote < ptr && ptr[-1] == '^';
             ptr--)
                len++;
        if (len)
                early = 1;
        else {
                early = 0;
                ptr = strrchr(remote, '~');
                if (ptr) {
                        int seen_nonzero = 0;

                        len++; /* count ~ */
                        while (*++ptr && isdigit(*ptr)) {
                                seen_nonzero |= (*ptr != '0');
                                len++;
                        }
                        if (*ptr)
                                len = 0; /* not ...~<number> */
                        else if (seen_nonzero)
                                early = 1;
                        else if (len == 1)
                                early = 1; /* "name~" is "name~1"! */
                }
        }
        if (len) {
                struct strbuf truname = STRBUF_INIT;
                strbuf_addf(&truname, "refs/heads/%s", remote);
                strbuf_setlen(&truname, truname.len - len);
                if (ref_exists(truname.buf)) {
                        strbuf_addf(msg,
                                    "%s\t\tbranch '%s'%s of .\n",
                                    oid_to_hex(&remote_head->object.oid),
                                    truname.buf + 11,
                                    (early ? " (early part)" : ""));
                        strbuf_release(&truname);
                        goto cleanup;
                }
                strbuf_release(&truname);
        }

        desc = merge_remote_util(remote_head);
        if (desc && desc->obj && desc->obj->type == OBJ_TAG) {
                strbuf_addf(msg, "%s\t\t%s '%s'\n",
                            oid_to_hex(&desc->obj->oid),
                            type_name(desc->obj->type),
                            remote);
                goto cleanup;
        }

        strbuf_addf(msg, "%s\t\tcommit '%s'\n",
                oid_to_hex(&remote_head->object.oid), remote);
cleanup:
        free(found_ref);
        strbuf_release(&bname);
}

static void parse_branch_merge_options(char *bmo)
{
        const char **argv;
        int argc;

        if (!bmo)
                return;
        argc = split_cmdline(bmo, &argv);
        if (argc < 0)
                die(_("Bad branch.%s.mergeoptions string: %s"), branch,
                    _(split_cmdline_strerror(argc)));
        REALLOC_ARRAY(argv, argc + 2);
        MOVE_ARRAY(argv + 1, argv, argc + 1);
        argc++;
        argv[0] = "branch.*.mergeoptions";
        parse_options(argc, argv, NULL, builtin_merge_options,
                      builtin_merge_usage, 0);
        free(argv);
}

static int git_merge_config(const char *k, const char *v,
                            const struct config_context *ctx, void *cb)
{
        int status;
        const char *str;

        if (branch &&
            skip_prefix(k, "branch.", &str) &&
            skip_prefix(str, branch, &str) &&
            !strcmp(str, ".mergeoptions")) {
                free(branch_mergeoptions);
                branch_mergeoptions = xstrdup(v);
                return 0;
        }

        if (!strcmp(k, "merge.diffstat") || !strcmp(k, "merge.stat"))
                show_diffstat = git_config_bool(k, v);
        else if (!strcmp(k, "merge.verifysignatures"))
                verify_signatures = git_config_bool(k, v);
        else if (!strcmp(k, "pull.twohead"))
                return git_config_string(&pull_twohead, k, v);
        else if (!strcmp(k, "pull.octopus"))
                return git_config_string(&pull_octopus, k, v);
        else if (!strcmp(k, "commit.cleanup"))
                return git_config_string(&cleanup_arg, k, v);
        else if (!strcmp(k, "merge.ff")) {
                int boolval = git_parse_maybe_bool(v);
                if (0 <= boolval) {
                        fast_forward = boolval ? FF_ALLOW : FF_NO;
                } else if (v && !strcmp(v, "only")) {
                        fast_forward = FF_ONLY;
                } /* do not barf on values from future versions of git */
                return 0;
        } else if (!strcmp(k, "merge.defaulttoupstream")) {
                default_to_upstream = git_config_bool(k, v);
                return 0;
        } else if (!strcmp(k, "commit.gpgsign")) {
                sign_commit = git_config_bool(k, v) ? "" : NULL;
                return 0;
        } else if (!strcmp(k, "gpg.mintrustlevel")) {
                check_trust_level = 0;
        } else if (!strcmp(k, "merge.autostash")) {
                autostash = git_config_bool(k, v);
                return 0;
        }

        status = fmt_merge_msg_config(k, v, ctx, cb);
        if (status)
                return status;
        return git_diff_ui_config(k, v, ctx, cb);
}

static int read_tree_trivial(struct object_id *common, struct object_id *head,
                             struct object_id *one)
{
        int i, nr_trees = 0;
        struct tree *trees[MAX_UNPACK_TREES];
        struct tree_desc t[MAX_UNPACK_TREES];
        struct unpack_trees_options opts;

        memset(&opts, 0, sizeof(opts));
        opts.head_idx = 2;
        opts.src_index = &the_index;
        opts.dst_index = &the_index;
        opts.update = 1;
        opts.verbose_update = 1;
        opts.trivial_merges_only = 1;
        opts.merge = 1;
        opts.preserve_ignored = 0; /* FIXME: !overwrite_ignore */
        trees[nr_trees] = parse_tree_indirect(common);
        if (!trees[nr_trees++])
                return -1;
        trees[nr_trees] = parse_tree_indirect(head);
        if (!trees[nr_trees++])
                return -1;
        trees[nr_trees] = parse_tree_indirect(one);
        if (!trees[nr_trees++])
                return -1;
        opts.fn = threeway_merge;
        cache_tree_free(&the_index.cache_tree);
        for (i = 0; i < nr_trees; i++) {
                parse_tree(trees[i]);
                init_tree_desc(t+i, trees[i]->buffer, trees[i]->size);
        }
        if (unpack_trees(nr_trees, t, &opts))
                return -1;
        return 0;
}

static void write_tree_trivial(struct object_id *oid)
{
        if (write_index_as_tree(oid, &the_index, get_index_file(), 0, NULL))
                die(_("git write-tree failed to write a tree"));
}

static int try_merge_strategy(const char *strategy, struct commit_list *common,
                              struct commit_list *remoteheads,
                              struct commit *head)
{
        const char *head_arg = "HEAD";

        if (repo_refresh_and_write_index(the_repository, REFRESH_QUIET,
                                         SKIP_IF_UNCHANGED, 0, NULL, NULL,
                                         NULL) < 0)
                return error(_("Unable to write index."));

        if (!strcmp(strategy, "recursive") || !strcmp(strategy, "subtree") ||
            !strcmp(strategy, "ort")) {
                struct lock_file lock = LOCK_INIT;
                int clean, x;
                struct commit *result;
                struct commit_list *reversed = NULL;
                struct merge_options o;
                struct commit_list *j;

                if (remoteheads->next) {
                        error(_("Not handling anything other than two heads merge."));
                        return 2;
                }

                init_merge_options(&o, the_repository);
                if (!strcmp(strategy, "subtree"))
                        o.subtree_shift = "";

                o.show_rename_progress =
                        show_progress == -1 ? isatty(2) : show_progress;

                for (x = 0; x < xopts.nr; x++)
                        if (parse_merge_opt(&o, xopts.v[x]))
                                die(_("unknown strategy option: -X%s"), xopts.v[x]);

                o.branch1 = head_arg;
                o.branch2 = merge_remote_util(remoteheads->item)->name;

                for (j = common; j; j = j->next)
                        commit_list_insert(j->item, &reversed);

                repo_hold_locked_index(the_repository, &lock,
                                       LOCK_DIE_ON_ERROR);
                if (!strcmp(strategy, "ort"))
                        clean = merge_ort_recursive(&o, head, remoteheads->item,
                                                    reversed, &result);
                else
                        clean = merge_recursive(&o, head, remoteheads->item,
                                                reversed, &result);
                if (clean < 0) {
                        rollback_lock_file(&lock);
                        return 2;
                }
                if (write_locked_index(&the_index, &lock,
                                       COMMIT_LOCK | SKIP_IF_UNCHANGED))
                        die(_("unable to write %s"), get_index_file());
                return clean ? 0 : 1;
        } else {
                return try_merge_command(the_repository,
                                         strategy, xopts.nr, xopts.v,
                                         common, head_arg, remoteheads);
        }
}

static void count_diff_files(struct diff_queue_struct *q,
                             struct diff_options *opt UNUSED, void *data)
{
        int *count = data;

        (*count) += q->nr;
}

static int count_unmerged_entries(void)
{
        int i, ret = 0;

        for (i = 0; i < the_index.cache_nr; i++)
                if (ce_stage(the_index.cache[i]))
                        ret++;

        return ret;
}

static void add_strategies(const char *string, unsigned attr)
{
        int i;

        if (string) {
                struct string_list list = STRING_LIST_INIT_DUP;
                struct string_list_item *item;
                string_list_split(&list, string, ' ', -1);
                for_each_string_list_item(item, &list)
                        append_strategy(get_strategy(item->string));
                string_list_clear(&list, 0);
                return;
        }
        for (i = 0; i < ARRAY_SIZE(all_strategy); i++)
                if (all_strategy[i].attr & attr)
                        append_strategy(&all_strategy[i]);

}

static void read_merge_msg(struct strbuf *msg)
{
        const char *filename = git_path_merge_msg(the_repository);
        strbuf_reset(msg);
        if (strbuf_read_file(msg, filename, 0) < 0)
                die_errno(_("Could not read from '%s'"), filename);
}

static void write_merge_state(struct commit_list *);
static void abort_commit(struct commit_list *remoteheads, const char *err_msg)
{
        if (err_msg)
                error("%s", err_msg);
        fprintf(stderr,
                _("Not committing merge; use 'git commit' to complete the merge.\n"));
        write_merge_state(remoteheads);
        exit(1);
}

static const char merge_editor_comment[] =
N_("Please enter a commit message to explain why this merge is necessary,\n"
   "especially if it merges an updated upstream into a topic branch.\n"
   "\n");

static const char scissors_editor_comment[] =
N_("An empty message aborts the commit.\n");

static const char no_scissors_editor_comment[] =
N_("Lines starting with '%c' will be ignored, and an empty message aborts\n"
   "the commit.\n");

static void write_merge_heads(struct commit_list *);
static void prepare_to_commit(struct commit_list *remoteheads)
{
        struct strbuf msg = STRBUF_INIT;
        const char *index_file = get_index_file();

        if (!no_verify) {
                int invoked_hook;

                if (run_commit_hook(0 < option_edit, index_file, &invoked_hook,
                                    "pre-merge-commit", NULL))
                        abort_commit(remoteheads, NULL);
                /*
                 * Re-read the index as pre-merge-commit hook could have updated it,
                 * and write it out as a tree.  We must do this before we invoke
                 * the editor and after we invoke run_status above.
                 */
                if (invoked_hook)
                        discard_index(&the_index);
        }
        read_index_from(&the_index, index_file, get_git_dir());
        strbuf_addbuf(&msg, &merge_msg);
        if (squash)
                BUG("the control must not reach here under --squash");
        if (0 < option_edit) {
                strbuf_addch(&msg, '\n');
                if (cleanup_mode == COMMIT_MSG_CLEANUP_SCISSORS) {
                        wt_status_append_cut_line(&msg);
                        strbuf_commented_addf(&msg, comment_line_char, "\n");
                }
                strbuf_commented_addf(&msg, comment_line_char,
                                      _(merge_editor_comment));
                if (cleanup_mode == COMMIT_MSG_CLEANUP_SCISSORS)
                        strbuf_commented_addf(&msg, comment_line_char,
                                              _(scissors_editor_comment));
                else
                        strbuf_commented_addf(&msg, comment_line_char,
                                _(no_scissors_editor_comment), comment_line_char);
        }
        if (signoff)
                append_signoff(&msg, ignored_log_message_bytes(msg.buf, msg.len), 0);
        write_merge_heads(remoteheads);
        write_file_buf(git_path_merge_msg(the_repository), msg.buf, msg.len);
        if (run_commit_hook(0 < option_edit, get_index_file(), NULL,
                            "prepare-commit-msg",
                            git_path_merge_msg(the_repository), "merge", NULL))
                abort_commit(remoteheads, NULL);
        if (0 < option_edit) {
                if (launch_editor(git_path_merge_msg(the_repository), NULL, NULL))
                        abort_commit(remoteheads, NULL);
        }

        if (!no_verify && run_commit_hook(0 < option_edit, get_index_file(),
                                          NULL, "commit-msg",
                                          git_path_merge_msg(the_repository), NULL))
                abort_commit(remoteheads, NULL);

        read_merge_msg(&msg);
        cleanup_message(&msg, cleanup_mode, 0);
        if (!msg.len)
                abort_commit(remoteheads, _("Empty commit message."));
        strbuf_release(&merge_msg);
        strbuf_addbuf(&merge_msg, &msg);
        strbuf_release(&msg);
}

static int merge_trivial(struct commit *head, struct commit_list *remoteheads)
{
        struct object_id result_tree, result_commit;
        struct commit_list *parents, **pptr = &parents;

        if (repo_refresh_and_write_index(the_repository, REFRESH_QUIET,
                                         SKIP_IF_UNCHANGED, 0, NULL, NULL,
                                         NULL) < 0)
                return error(_("Unable to write index."));

        write_tree_trivial(&result_tree);
        printf(_("Wonderful.\n"));
        pptr = commit_list_append(head, pptr);
        pptr = commit_list_append(remoteheads->item, pptr);
        prepare_to_commit(remoteheads);
        if (commit_tree(merge_msg.buf, merge_msg.len, &result_tree, parents,
                        &result_commit, NULL, sign_commit))
                die(_("failed to write commit object"));
        finish(head, remoteheads, &result_commit, "In-index merge");
        remove_merge_branch_state(the_repository);
        return 0;
}

static int finish_automerge(struct commit *head,
                            int head_subsumed,
                            struct commit_list *common,
                            struct commit_list *remoteheads,
                            struct object_id *result_tree,
                            const char *wt_strategy)
{
        struct commit_list *parents = NULL;
        struct strbuf buf = STRBUF_INIT;
        struct object_id result_commit;

        write_tree_trivial(result_tree);
        free_commit_list(common);
        parents = remoteheads;
        if (!head_subsumed || fast_forward == FF_NO)
                commit_list_insert(head, &parents);
        prepare_to_commit(remoteheads);
        if (commit_tree(merge_msg.buf, merge_msg.len, result_tree, parents,
                        &result_commit, NULL, sign_commit))
                die(_("failed to write commit object"));
        strbuf_addf(&buf, "Merge made by the '%s' strategy.", wt_strategy);
        finish(head, remoteheads, &result_commit, buf.buf);
        strbuf_release(&buf);
        remove_merge_branch_state(the_repository);
        return 0;
}

static int suggest_conflicts(void)
{
        const char *filename;
        FILE *fp;
        struct strbuf msgbuf = STRBUF_INIT;

        filename = git_path_merge_msg(the_repository);
        fp = xfopen(filename, "a");

        /*
         * We can't use cleanup_mode because if we're not using the editor,
         * get_cleanup_mode will return COMMIT_MSG_CLEANUP_SPACE instead, even
         * though the message is meant to be processed later by git-commit.
         * Thus, we will get the cleanup mode which is returned when we _are_
         * using an editor.
         */
        append_conflicts_hint(&the_index, &msgbuf,
                              get_cleanup_mode(cleanup_arg, 1));
        fputs(msgbuf.buf, fp);
        strbuf_release(&msgbuf);
        fclose(fp);
        repo_rerere(the_repository, allow_rerere_auto);
        printf(_("Automatic merge failed; "
                        "fix conflicts and then commit the result.\n"));
        return 1;
}

static int evaluate_result(void)
{
        int cnt = 0;
        struct rev_info rev;

        /* Check how many files differ. */
        repo_init_revisions(the_repository, &rev, "");
        setup_revisions(0, NULL, &rev, NULL);
        rev.diffopt.output_format |=
                DIFF_FORMAT_CALLBACK;
        rev.diffopt.format_callback = count_diff_files;
        rev.diffopt.format_callback_data = &cnt;
        run_diff_files(&rev, 0);

        /*
         * Check how many unmerged entries are
         * there.
         */
        cnt += count_unmerged_entries();

        release_revisions(&rev);
        return cnt;
}

/*
 * Pretend as if the user told us to merge with the remote-tracking
 * branch we have for the upstream of the current branch
 */
static int setup_with_upstream(const char ***argv)
{
        struct branch *branch = branch_get(NULL);
        int i;
        const char **args;

        if (!branch)
                die(_("No current branch."));
        if (!branch->remote_name)
                die(_("No remote for the current branch."));
        if (!branch->merge_nr)
                die(_("No default upstream defined for the current branch."));

        args = xcalloc(st_add(branch->merge_nr, 1), sizeof(char *));
        for (i = 0; i < branch->merge_nr; i++) {
                if (!branch->merge[i]->dst)
                        die(_("No remote-tracking branch for %s from %s"),
                            branch->merge[i]->src, branch->remote_name);
                args[i] = branch->merge[i]->dst;
        }
        args[i] = NULL;
        *argv = args;
        return i;
}

static void write_merge_heads(struct commit_list *remoteheads)
{
        struct commit_list *j;
        struct strbuf buf = STRBUF_INIT;

        for (j = remoteheads; j; j = j->next) {
                struct object_id *oid;
                struct commit *c = j->item;
                struct merge_remote_desc *desc;

                desc = merge_remote_util(c);
                if (desc && desc->obj) {
                        oid = &desc->obj->oid;
                } else {
                        oid = &c->object.oid;
                }
                strbuf_addf(&buf, "%s\n", oid_to_hex(oid));
        }
        write_file_buf(git_path_merge_head(the_repository), buf.buf, buf.len);

        strbuf_reset(&buf);
        if (fast_forward == FF_NO)
                strbuf_addstr(&buf, "no-ff");
        write_file_buf(git_path_merge_mode(the_repository), buf.buf, buf.len);
        strbuf_release(&buf);
}

static void write_merge_state(struct commit_list *remoteheads)
{
        write_merge_heads(remoteheads);
        strbuf_addch(&merge_msg, '\n');
        write_file_buf(git_path_merge_msg(the_repository), merge_msg.buf,
                       merge_msg.len);
}

static int default_edit_option(void)
{
        static const char name[] = "GIT_MERGE_AUTOEDIT";
        const char *e = getenv(name);
        struct stat st_stdin, st_stdout;

        if (have_message)
                /* an explicit -m msg without --[no-]edit */
                return 0;

        if (e) {
                int v = git_parse_maybe_bool(e);
                if (v < 0)
                        die(_("Bad value '%s' in environment '%s'"), e, name);
                return v;
        }

        /* Use editor if stdin and stdout are the same and is a tty */
        return (!fstat(0, &st_stdin) &&
                !fstat(1, &st_stdout) &&
                isatty(0) && isatty(1) &&
                st_stdin.st_dev == st_stdout.st_dev &&
                st_stdin.st_ino == st_stdout.st_ino &&
                st_stdin.st_mode == st_stdout.st_mode);
}

static struct commit_list *reduce_parents(struct commit *head_commit,
                                          int *head_subsumed,
                                          struct commit_list *remoteheads)
{
        struct commit_list *parents, **remotes;

        /*
         * Is the current HEAD reachable from another commit being
         * merged?  If so we do not want to record it as a parent of
         * the resulting merge, unless --no-ff is given.  We will flip
         * this variable to 0 when we find HEAD among the independent
         * tips being merged.
         */
        *head_subsumed = 1;

        /* Find what parents to record by checking independent ones. */
        parents = reduce_heads(remoteheads);
        free_commit_list(remoteheads);

        remoteheads = NULL;
        remotes = &remoteheads;
        while (parents) {
                struct commit *commit = pop_commit(&parents);
                if (commit == head_commit)
                        *head_subsumed = 0;
                else
                        remotes = &commit_list_insert(commit, remotes)->next;
        }
        return remoteheads;
}

static void prepare_merge_message(struct strbuf *merge_names, struct strbuf *merge_msg)
{
        struct fmt_merge_msg_opts opts;

        memset(&opts, 0, sizeof(opts));
        opts.add_title = !have_message;
        opts.shortlog_len = shortlog_len;
        opts.credit_people = (0 < option_edit);
        opts.into_name = into_name;

        fmt_merge_msg(merge_names, merge_msg, &opts);
        if (merge_msg->len)
                strbuf_setlen(merge_msg, merge_msg->len - 1);
}

static void handle_fetch_head(struct commit_list **remotes, struct strbuf *merge_names)
{
        const char *filename;
        int fd, pos, npos;
        struct strbuf fetch_head_file = STRBUF_INIT;
        const unsigned hexsz = the_hash_algo->hexsz;

        if (!merge_names)
                merge_names = &fetch_head_file;

        filename = git_path_fetch_head(the_repository);
        fd = xopen(filename, O_RDONLY);

        if (strbuf_read(merge_names, fd, 0) < 0)
                die_errno(_("could not read '%s'"), filename);
        if (close(fd) < 0)
                die_errno(_("could not close '%s'"), filename);

        for (pos = 0; pos < merge_names->len; pos = npos) {
                struct object_id oid;
                char *ptr;
                struct commit *commit;

                ptr = strchr(merge_names->buf + pos, '\n');
                if (ptr)
                        npos = ptr - merge_names->buf + 1;
                else
                        npos = merge_names->len;

                if (npos - pos < hexsz + 2 ||
                    get_oid_hex(merge_names->buf + pos, &oid))
                        commit = NULL; /* bad */
                else if (memcmp(merge_names->buf + pos + hexsz, "\t\t", 2))
                        continue; /* not-for-merge */
                else {
                        char saved = merge_names->buf[pos + hexsz];
                        merge_names->buf[pos + hexsz] = '\0';
                        commit = get_merge_parent(merge_names->buf + pos);
                        merge_names->buf[pos + hexsz] = saved;
                }
                if (!commit) {
                        if (ptr)
                                *ptr = '\0';
                        die(_("not something we can merge in %s: %s"),
                            filename, merge_names->buf + pos);
                }
                remotes = &commit_list_insert(commit, remotes)->next;
        }

        if (merge_names == &fetch_head_file)
                strbuf_release(&fetch_head_file);
}

static struct commit_list *collect_parents(struct commit *head_commit,
                                           int *head_subsumed,
                                           int argc, const char **argv,
                                           struct strbuf *merge_msg)
{
        int i;
        struct commit_list *remoteheads = NULL;
        struct commit_list **remotes = &remoteheads;
        struct strbuf merge_names = STRBUF_INIT, *autogen = NULL;

        if (merge_msg && (!have_message || shortlog_len))
                autogen = &merge_names;

        if (head_commit)
                remotes = &commit_list_insert(head_commit, remotes)->next;

        if (argc == 1 && !strcmp(argv[0], "FETCH_HEAD")) {
                handle_fetch_head(remotes, autogen);
                remoteheads = reduce_parents(head_commit, head_subsumed, remoteheads);
        } else {
                for (i = 0; i < argc; i++) {
                        struct commit *commit = get_merge_parent(argv[i]);
                        if (!commit)
                                help_unknown_ref(argv[i], "merge",
                                                 _("not something we can merge"));
                        remotes = &commit_list_insert(commit, remotes)->next;
                }
                remoteheads = reduce_parents(head_commit, head_subsumed, remoteheads);
                if (autogen) {
                        struct commit_list *p;
                        for (p = remoteheads; p; p = p->next)
                                merge_name(merge_remote_util(p->item)->name, autogen);
                }
        }

        if (autogen) {
                prepare_merge_message(autogen, merge_msg);
                strbuf_release(autogen);
        }

        return remoteheads;
}

static int merging_a_throwaway_tag(struct commit *commit)
{
        char *tag_ref;
        struct object_id oid;
        int is_throwaway_tag = 0;

        /* Are we merging a tag? */
        if (!merge_remote_util(commit) ||
            !merge_remote_util(commit)->obj ||
            merge_remote_util(commit)->obj->type != OBJ_TAG)
                return is_throwaway_tag;

        /*
         * Now we know we are merging a tag object.  Are we downstream
         * and following the tags from upstream?  If so, we must have
         * the tag object pointed at by "refs/tags/$T" where $T is the
         * tagname recorded in the tag object.  We want to allow such
         * a "just to catch up" merge to fast-forward.
         *
         * Otherwise, we are playing an integrator's role, making a
         * merge with a throw-away tag from a contributor with
         * something like "git pull $contributor $signed_tag".
         * We want to forbid such a merge from fast-forwarding
         * by default; otherwise we would not keep the signature
         * anywhere.
         */
        tag_ref = xstrfmt("refs/tags/%s",
                          ((struct tag *)merge_remote_util(commit)->obj)->tag);
        if (!read_ref(tag_ref, &oid) &&
            oideq(&oid, &merge_remote_util(commit)->obj->oid))
                is_throwaway_tag = 0;
        else
                is_throwaway_tag = 1;
        free(tag_ref);
        return is_throwaway_tag;
}

int cmd_merge(int argc, const char **argv, const char *prefix)
{
        struct object_id result_tree, stash, head_oid;
        struct commit *head_commit;
        struct strbuf buf = STRBUF_INIT;
        int i, ret = 0, head_subsumed;
        int best_cnt = -1, merge_was_ok = 0, automerge_was_ok = 0;
        struct commit_list *common = NULL;
        const char *best_strategy = NULL, *wt_strategy = NULL;
        struct commit_list *remoteheads = NULL, *p;
        void *branch_to_free;
        int orig_argc = argc;

        if (argc == 2 && !strcmp(argv[1], "-h"))
                usage_with_options(builtin_merge_usage, builtin_merge_options);

        prepare_repo_settings(the_repository);
        the_repository->settings.command_requires_full_index = 0;

        /*
         * Check if we are _not_ on a detached HEAD, i.e. if there is a
         * current branch.
         */
        branch = branch_to_free = resolve_refdup("HEAD", 0, &head_oid, NULL);
        if (branch)
                skip_prefix(branch, "refs/heads/", &branch);

        if (!pull_twohead) {
                char *default_strategy = getenv("GIT_TEST_MERGE_ALGORITHM");
                if (default_strategy && !strcmp(default_strategy, "ort"))
                        pull_twohead = "ort";
        }

        init_diff_ui_defaults();
        git_config(git_merge_config, NULL);

        if (!branch || is_null_oid(&head_oid))
                head_commit = NULL;
        else
                head_commit = lookup_commit_or_die(&head_oid, "HEAD");

        if (branch_mergeoptions)
                parse_branch_merge_options(branch_mergeoptions);
        argc = parse_options(argc, argv, prefix, builtin_merge_options,
                        builtin_merge_usage, 0);
        if (shortlog_len < 0)
                shortlog_len = (merge_log_config > 0) ? merge_log_config : 0;

        if (verbosity < 0 && show_progress == -1)
                show_progress = 0;

        if (abort_current_merge) {
                int nargc = 2;
                const char *nargv[] = {"reset", "--merge", NULL};
                char stash_oid_hex[GIT_MAX_HEXSZ + 1];
                struct object_id stash_oid = {0};

                if (orig_argc != 2)
                        usage_msg_opt(_("--abort expects no arguments"),
                              builtin_merge_usage, builtin_merge_options);

                if (!file_exists(git_path_merge_head(the_repository)))
                        die(_("There is no merge to abort (MERGE_HEAD missing)."));

                if (!read_ref("MERGE_AUTOSTASH", &stash_oid))
                        delete_ref("", "MERGE_AUTOSTASH", &stash_oid, REF_NO_DEREF);

                /* Invoke 'git reset --merge' */
                ret = cmd_reset(nargc, nargv, prefix);

                if (!is_null_oid(&stash_oid)) {
                        oid_to_hex_r(stash_oid_hex, &stash_oid);
                        apply_autostash_oid(stash_oid_hex);
                }

                goto done;
        }

        if (quit_current_merge) {
                if (orig_argc != 2)
                        usage_msg_opt(_("--quit expects no arguments"),
                                      builtin_merge_usage,
                                      builtin_merge_options);

                remove_merge_branch_state(the_repository);
                goto done;
        }

        if (continue_current_merge) {
                int nargc = 1;
                const char *nargv[] = {"commit", NULL};

                if (orig_argc != 2)
                        usage_msg_opt(_("--continue expects no arguments"),
                              builtin_merge_usage, builtin_merge_options);

                if (!file_exists(git_path_merge_head(the_repository)))
                        die(_("There is no merge in progress (MERGE_HEAD missing)."));

                /* Invoke 'git commit' */
                ret = cmd_commit(nargc, nargv, prefix);
                goto done;
        }

        if (repo_read_index_unmerged(the_repository))
                die_resolve_conflict("merge");

        if (file_exists(git_path_merge_head(the_repository))) {
                /*
                 * There is no unmerged entry, don't advise 'git
                 * add/rm <file>', just 'git commit'.
                 */
                if (advice_enabled(ADVICE_RESOLVE_CONFLICT))
                        die(_("You have not concluded your merge (MERGE_HEAD exists).\n"
                                  "Please, commit your changes before you merge."));
                else
                        die(_("You have not concluded your merge (MERGE_HEAD exists)."));
        }
        if (ref_exists("CHERRY_PICK_HEAD")) {
                if (advice_enabled(ADVICE_RESOLVE_CONFLICT))
                        die(_("You have not concluded your cherry-pick (CHERRY_PICK_HEAD exists).\n"
                            "Please, commit your changes before you merge."));
                else
                        die(_("You have not concluded your cherry-pick (CHERRY_PICK_HEAD exists)."));
        }
        resolve_undo_clear_index(&the_index);

        if (option_edit < 0)
                option_edit = default_edit_option();

        cleanup_mode = get_cleanup_mode(cleanup_arg, 0 < option_edit);

        if (verbosity < 0)
                show_diffstat = 0;

        if (squash) {
                if (fast_forward == FF_NO)
                        die(_("options '%s' and '%s' cannot be used together"), "--squash", "--no-ff.");
                if (option_commit > 0)
                        die(_("options '%s' and '%s' cannot be used together"), "--squash", "--commit.");
                /*
                 * squash can now silently disable option_commit - this is not
                 * a problem as it is only overriding the default, not a user
                 * supplied option.
                 */
                option_commit = 0;
        }

        if (option_commit < 0)
                option_commit = 1;

        if (!argc) {
                if (default_to_upstream)
                        argc = setup_with_upstream(&argv);
                else
                        die(_("No commit specified and merge.defaultToUpstream not set."));
        } else if (argc == 1 && !strcmp(argv[0], "-")) {
                argv[0] = "@{-1}";
        }

        if (!argc)
                usage_with_options(builtin_merge_usage,
                        builtin_merge_options);

        if (!head_commit) {
                /*
                 * If the merged head is a valid one there is no reason
                 * to forbid "git merge" into a branch yet to be born.
                 * We do the same for "git pull".
                 */
                struct object_id *remote_head_oid;
                if (squash)
                        die(_("Squash commit into empty head not supported yet"));
                if (fast_forward == FF_NO)
                        die(_("Non-fast-forward commit does not make sense into "
                            "an empty head"));
                remoteheads = collect_parents(head_commit, &head_subsumed,
                                              argc, argv, NULL);
                if (!remoteheads)
                        die(_("%s - not something we can merge"), argv[0]);
                if (remoteheads->next)
                        die(_("Can merge only exactly one commit into empty head"));

                if (verify_signatures)
                        verify_merge_signature(remoteheads->item, verbosity,
                                               check_trust_level);

                remote_head_oid = &remoteheads->item->object.oid;
                read_empty(remote_head_oid);
                update_ref("initial pull", "HEAD", remote_head_oid, NULL, 0,
                           UPDATE_REFS_DIE_ON_ERR);
                goto done;
        }

        /*
         * All the rest are the commits being merged; prepare
         * the standard merge summary message to be appended
         * to the given message.
         */
        remoteheads = collect_parents(head_commit, &head_subsumed,
                                      argc, argv, &merge_msg);

        if (!head_commit || !argc)
                usage_with_options(builtin_merge_usage,
                        builtin_merge_options);

        if (verify_signatures) {
                for (p = remoteheads; p; p = p->next) {
                        verify_merge_signature(p->item, verbosity,
                                               check_trust_level);
                }
        }

        strbuf_addstr(&buf, "merge");
        for (p = remoteheads; p; p = p->next)
                strbuf_addf(&buf, " %s", merge_remote_util(p->item)->name);
        setenv("GIT_REFLOG_ACTION", buf.buf, 0);
        strbuf_reset(&buf);

        for (p = remoteheads; p; p = p->next) {
                struct commit *commit = p->item;
                strbuf_addf(&buf, "GITHEAD_%s",
                            oid_to_hex(&commit->object.oid));
                setenv(buf.buf, merge_remote_util(commit)->name, 1);
                strbuf_reset(&buf);
                if (fast_forward != FF_ONLY && merging_a_throwaway_tag(commit))
                        fast_forward = FF_NO;
        }

        if (!use_strategies && !pull_twohead &&
            remoteheads && !remoteheads->next) {
                char *default_strategy = getenv("GIT_TEST_MERGE_ALGORITHM");
                if (default_strategy)
                        append_strategy(get_strategy(default_strategy));
        }
        if (!use_strategies) {
                if (!remoteheads)
                        ; /* already up-to-date */
                else if (!remoteheads->next)
                        add_strategies(pull_twohead, DEFAULT_TWOHEAD);
                else
                        add_strategies(pull_octopus, DEFAULT_OCTOPUS);
        }

        for (i = 0; i < use_strategies_nr; i++) {
                if (use_strategies[i]->attr & NO_FAST_FORWARD)
                        fast_forward = FF_NO;
                if (use_strategies[i]->attr & NO_TRIVIAL)
                        allow_trivial = 0;
        }

        if (!remoteheads)
                ; /* already up-to-date */
        else if (!remoteheads->next) {
                if (repo_get_merge_bases(the_repository, head_commit,
                                         remoteheads->item, &common) < 0) {
                        ret = 2;
                        goto done;
                }
        } else {
                struct commit_list *list = remoteheads;
                commit_list_insert(head_commit, &list);
                if (get_octopus_merge_bases(list, &common) < 0) {
                        free(list);
                        ret = 2;
                        goto done;
                }
                free(list);
        }

        update_ref("updating ORIG_HEAD", "ORIG_HEAD",
                   &head_commit->object.oid, NULL, 0, UPDATE_REFS_DIE_ON_ERR);

        if (remoteheads && !common) {
                /* No common ancestors found. */
                if (!allow_unrelated_histories)
                        die(_("refusing to merge unrelated histories"));
                /* otherwise, we need a real merge. */
        } else if (!remoteheads ||
                 (!remoteheads->next && !common->next &&
                  common->item == remoteheads->item)) {
                /*
                 * If head can reach all the merge then we are up to date.
                 * but first the most common case of merging one remote.
                 */
                finish_up_to_date();
                goto done;
        } else if (fast_forward != FF_NO && !remoteheads->next &&
                        !common->next &&
                        oideq(&common->item->object.oid, &head_commit->object.oid)) {
                /* Again the most common case of merging one remote. */
                const char *msg = have_message ?
                        "Fast-forward (no commit created; -m option ignored)" :
                        "Fast-forward";
                struct commit *commit;

                if (verbosity >= 0) {
                        printf(_("Updating %s..%s\n"),
                               repo_find_unique_abbrev(the_repository, &head_commit->object.oid,
                                                       DEFAULT_ABBREV),
                               repo_find_unique_abbrev(the_repository, &remoteheads->item->object.oid,
                                                       DEFAULT_ABBREV));
                }
                commit = remoteheads->item;
                if (!commit) {
                        ret = 1;
                        goto done;
                }

                if (autostash)
                        create_autostash_ref(the_repository, "MERGE_AUTOSTASH");
                if (checkout_fast_forward(the_repository,
                                          &head_commit->object.oid,
                                          &commit->object.oid,
                                          overwrite_ignore)) {
                        apply_autostash_ref(the_repository, "MERGE_AUTOSTASH");
                        ret = 1;
                        goto done;
                }

                finish(head_commit, remoteheads, &commit->object.oid, msg);
                remove_merge_branch_state(the_repository);
                goto done;
        } else if (!remoteheads->next && common->next)
                ;
                /*
                 * We are not doing octopus and not fast-forward.  Need
                 * a real merge.
                 */
        else if (!remoteheads->next && !common->next && option_commit) {
                /*
                 * We are not doing octopus, not fast-forward, and have
                 * only one common.
                 */
                refresh_index(&the_index, REFRESH_QUIET, NULL, NULL, NULL);
                if (allow_trivial && fast_forward != FF_ONLY) {
                        /*
                         * Must first ensure that index matches HEAD before
                         * attempting a trivial merge.
                         */
                        struct tree *head_tree = repo_get_commit_tree(the_repository,
                                                                      head_commit);
                        struct strbuf sb = STRBUF_INIT;

                        if (repo_index_has_changes(the_repository, head_tree,
                                                   &sb)) {
                                error(_("Your local changes to the following files would be overwritten by merge:\n  %s"),
                                      sb.buf);
                                strbuf_release(&sb);
                                ret = 2;
                                goto done;
                        }

                        /* See if it is really trivial. */
                        git_committer_info(IDENT_STRICT);
                        printf(_("Trying really trivial in-index merge...\n"));
                        if (!read_tree_trivial(&common->item->object.oid,
                                               &head_commit->object.oid,
                                               &remoteheads->item->object.oid)) {
                                ret = merge_trivial(head_commit, remoteheads);
                                goto done;
                        }
                        printf(_("Nope.\n"));
                }
        } else {
                /*
                 * An octopus.  If we can reach all the remote we are up
                 * to date.
                 */
                int up_to_date = 1;
                struct commit_list *j;

                for (j = remoteheads; j; j = j->next) {
                        struct commit_list *common_one = NULL;
                        struct commit *common_item;

                        /*
                         * Here we *have* to calculate the individual
                         * merge_bases again, otherwise "git merge HEAD^
                         * HEAD^^" would be missed.
                         */
                        if (repo_get_merge_bases(the_repository, head_commit,
                                                 j->item, &common_one) < 0)
                                exit(128);

                        common_item = common_one->item;
                        free_commit_list(common_one);
                        if (!oideq(&common_item->object.oid, &j->item->object.oid)) {
                                up_to_date = 0;
                                break;
                        }
                }
                if (up_to_date) {
                        finish_up_to_date();
                        goto done;
                }
        }

        if (fast_forward == FF_ONLY)
                die_ff_impossible();

        if (autostash)
                create_autostash_ref(the_repository, "MERGE_AUTOSTASH");

        /* We are going to make a new commit. */
        git_committer_info(IDENT_STRICT);

        /*
         * At this point, we need a real merge.  No matter what strategy
         * we use, it would operate on the index, possibly affecting the
         * working tree, and when resolved cleanly, have the desired
         * tree in the index -- this means that the index must be in
         * sync with the head commit.  The strategies are responsible
         * to ensure this.
         *
         * Stash away the local changes so that we can try more than one
         * and/or recover from merge strategies bailing while leaving the
         * index and working tree polluted.
         */
        if (save_state(&stash))
                oidclr(&stash);

        for (i = 0; i < use_strategies_nr; i++) {
                int ret, cnt;
                if (i) {
                        printf(_("Rewinding the tree to pristine...\n"));
                        restore_state(&head_commit->object.oid, &stash);
                }
                if (use_strategies_nr != 1)
                        printf(_("Trying merge strategy %s...\n"),
                                use_strategies[i]->name);
                /*
                 * Remember which strategy left the state in the working
                 * tree.
                 */
                wt_strategy = use_strategies[i]->name;

                ret = try_merge_strategy(wt_strategy,
                                         common, remoteheads,
                                         head_commit);
                /*
                 * The backend exits with 1 when conflicts are
                 * left to be resolved, with 2 when it does not
                 * handle the given merge at all.
                 */
                if (ret < 2) {
                        if (!ret) {
                                /*
                                 * This strategy worked; no point in trying
                                 * another.
                                 */
                                merge_was_ok = 1;
                                best_strategy = wt_strategy;
                                break;
                        }
                        cnt = (use_strategies_nr > 1) ? evaluate_result() : 0;
                        if (best_cnt <= 0 || cnt <= best_cnt) {
                                best_strategy = wt_strategy;
                                best_cnt = cnt;
                        }
                }
        }

        /*
         * If we have a resulting tree, that means the strategy module
         * auto resolved the merge cleanly.
         */
        if (merge_was_ok && option_commit) {
                automerge_was_ok = 1;
                ret = finish_automerge(head_commit, head_subsumed,
                                       common, remoteheads,
                                       &result_tree, wt_strategy);
                goto done;
        }

        /*
         * Pick the result from the best strategy and have the user fix
         * it up.
         */
        if (!best_strategy) {
                restore_state(&head_commit->object.oid, &stash);
                if (use_strategies_nr > 1)
                        fprintf(stderr,
                                _("No merge strategy handled the merge.\n"));
                else
                        fprintf(stderr, _("Merge with strategy %s failed.\n"),
                                use_strategies[0]->name);
                apply_autostash_ref(the_repository, "MERGE_AUTOSTASH");
                ret = 2;
                goto done;
        } else if (best_strategy == wt_strategy)
                ; /* We already have its result in the working tree. */
        else {
                printf(_("Rewinding the tree to pristine...\n"));
                restore_state(&head_commit->object.oid, &stash);
                printf(_("Using the %s strategy to prepare resolving by hand.\n"),
                        best_strategy);
                try_merge_strategy(best_strategy, common, remoteheads,
                                   head_commit);
        }

        if (squash) {
                finish(head_commit, remoteheads, NULL, NULL);

                git_test_write_commit_graph_or_die();
        } else
                write_merge_state(remoteheads);

        if (merge_was_ok)
                fprintf(stderr, _("Automatic merge went well; "
                        "stopped before committing as requested\n"));
        else
                ret = suggest_conflicts();
        if (autostash)
                printf(_("When finished, apply stashed changes with `git stash pop`\n"));

done:
        if (!automerge_was_ok) {
                free_commit_list(common);
                free_commit_list(remoteheads);
        }
        strbuf_release(&buf);
        free(branch_to_free);
        discard_index(&the_index);
        return ret;
}