rebase: fix rewritten list for failed pick

rebase: fix rewritten list for failed pick

git rebase keeps a list that maps the OID of each commit before it was
rebased to the OID of the equivalent commit after the rebase.  This list
is used to drive the "post-rewrite" hook that is called at the end of a
successful rebase. When a rebase stops for the user to resolve merge
conflicts the OID of the commit being picked is written to
".git/rebase-merge/stopped-sha". Then when the rebase is continued that
OID is added to the list of rewritten commits. Unfortunately if a commit
cannot be picked because it would overwrite an untracked file we still
write the "stopped-sha1" file. This means that when the rebase is
continued the commit is added into the list of rewritten commits even
though it has not been picked yet.

Fix this by not calling error_with_patch() for failed commands. The pick
has failed so there is nothing to commit and therefore we do not want to
set up the state files for committing staged changes when the rebase
continues. This change means we no-longer write a patch for the failed
command or display the error message printed by error_with_patch(). As
the command has failed the patch isn't really useful and in any case the
user can inspect the commit associated with the failed command by
inspecting REBASE_HEAD. Unless the user has disabled it we already print
an advice message that is more helpful than the message from
error_with_patch() which the user will still see. Even if the advice is
disabled the user will see the messages from the merge machinery
detailing the problem.

The code to add a failed command back into the todo list is duplicated
between pick_one_commit() and the loop in pick_commits(). Both sites
print advice about the command being rescheduled, decrement the current
item and save the todo list. To avoid duplicating this code
pick_one_commit() is modified to set a flag to indicate that the command
should be rescheduled in the main loop. This simplifies things as only
the remaining copy of the code needs to be modified to set REBASE_HEAD
rather than calling error_with_patch().

Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>