From bde794827b895f1b7834688a7210ea04dd417fbc Mon Sep 17 00:00:00 2001
From: "Kyle J. McKay" <mackyle@gmail.com>
Date: Wed, 29 Mar 2017 23:01:15 -0700
Subject: [PATCH] tg-tag.sh: allow choosing a consolidation commit's first
 parent

When multiple branches are tagged, if there ends up being more than
one independent head, a consolidation commit will be created and
that will be tagged.

The previously provided `--tree` option allows the tree used for
that commit to be something other than the empty tree.

Now, if one of the things being tagged is explicitly prefixed
with a tilde ('~'), it will be forced to become the first parent
of a consolidation commit if it's not the sole commit being
tagged already and, in addition, if --tree has NOT been used
then it's tree will become the default tree for any consolidation
commit instead of the empty tree (but --tree is always respected
if given).

Signed-off-by: Kyle J. McKay <mackyle@gmail.com>
---
 README    | 10 ++++++++++
 tg-tag.sh | 31 ++++++++++++++++++++++++++-----
 2 files changed, 36 insertions(+), 5 deletions(-)

diff --git a/README b/README
index 2466d51..f8f8d21 100644
--- a/README
+++ b/README
@@ -1452,6 +1452,16 @@ tg tag
 	what's in the commit the newly created tag refers to).  The argument to
 	the ``--tree`` option may be any valid treeish.
 
+	If exactly one of the branches to be tagged is prefixed with a tilde
+	(``~``) it will be made the first parent of a consolidation commit if
+	it is not already the sole commit needing to be tagged.  If ``--tree``
+	is NOT used, its tree will also be used instead of the empty tree for
+	any new consolidation commit if one is created.  Note that if
+	``--tree`` is given explicitly it's tree is always used but that does
+	not in any way affect the choice of first parent.  Beware that the
+	``~`` may need to be quoted to prevent the shell from misinterpreting
+	it into something else.
+
 	All the options for creating a tag serve the same purpose as their Git
 	equivalents except for two.  The ``--refs`` option suppresses tag
 	creation entirely and emits the "TOPGIT REFS" section that would have
diff --git a/tg-tag.sh b/tg-tag.sh
index 976077a..91558c8 100644
--- a/tg-tag.sh
+++ b/tg-tag.sh
@@ -412,6 +412,14 @@ tgcount=0
 othercount=0
 ignore=
 newlist=
+firstprnt=
+for arg in "$@"; do
+	case "$arg" in "~"?*)
+		[ -z "$firstprnt" ] || die "only one first parent may be specified with ~"
+		firstprnt="$(git rev-parse --verify --quiet "${arg#?}^0" -- 2>/dev/null)" && [ -n "$firstprnt" ] ||
+			die "not a commit-ish: ${arg#?}"
+	esac
+done
 while read -r obj typ ref && [ -n "$obj" -a -n "$typ" ]; do
 	[ -n "$ref" -o "$typ" != "missing" ] || die "no such ref: ${obj%???}"
 	case " $ignore " in *" $ref "*) continue; esac
@@ -430,8 +438,8 @@ while read -r obj typ ref && [ -n "$obj" -a -n "$typ" ]; do
 	fi
 done <<-EOT
 	$({
-		printf '%s\n' "$@" | sed 's/^\(.*\)$/\1^{} \1/'
-		printf '%s\n' "$@" | sed 's/^\(.*\)$/\1 \1/'
+		printf '%s\n' "$@" | sed 's/^~//; s/^\(.*\)$/\1^{} \1/'
+		printf '%s\n' "$@" | sed 's/^~//; s/^\(.*\)$/\1 \1/'
 	} |
 	git cat-file --batch-check='%(objectname) %(objecttype) %(rest)' 2>/dev/null ||
 	:)
@@ -657,9 +665,22 @@ case "$allrefs${extrarefs:+ $extrarefs}" in
 			die "failed: git merge-base --independent"
 		;;
 	*)
-		parents="$allrefs^0"
+		if [ -n "$firstprnt" ]; then
+			parents="$(git rev-parse --quiet --verify "$allrefs^0" --)" ||
+				die "failed: git rev-parse $allrefs^0"
+		else
+			parents="$allrefs^0"
+		fi
 		;;
 esac
+if [ -n "$firstprnt" ]; then
+	oldparents="$parents"
+	parents="$firstprnt"
+	for acmt in $oldparents; do
+		[ "$acmt" = "$firstprnt" ] || parents="$parents $acmt"
+	done
+	unset oldparents
+fi
 v_count_args pcnt $parents
 if [ $pcnt -eq 1 ]; then
 	tagtarget="$parents"
@@ -668,10 +689,10 @@ if [ $pcnt -eq 1 ]; then
 	tagtarget=
 fi
 if [ -z "$tagtarget" ]; then
-	tagtree="$treeish"
+	tagtree="${treeish:-$firstprnt}"
 	[ -n "$tagtree" ] || tagtree="$(git hash-object -t tree -w --stdin </dev/null)"
 	tagtarget="$(printf '%s\n' "tg tag branch consolidation" "" $branches |
-		git commit-tree $tagtree $(printf -- '-p %s ' $parents))"
+		git commit-tree $tagtree^{tree} $(printf -- '-p %s ' $parents))"
 fi
 
 init_reflog "$refname"
-- 
2.11.4.GIT