sequencer.c: always free() the "msgbuf" in do_pick_commit()

In [1] the strbuf_release(&msgbuf) was moved into this
do_pick_commit(), but didn't take into account the case of [2], where
we'd return before the strbuf_release(&msgbuf).

Then when the "fixup" support was added in [3] this leak got worse, as
in this error case we added another place where we'd "return" before
reaching the strbuf_release().

This changes the behavior so that we'll call
update_abort_safety_file() in these cases where we'd previously
"return", but as noted in [4] "update_abort_safety_file() is a no-op
when rebasing and you're changing code that is only run when
rebasing.". Here "no-op" refers to the early return in
update_abort_safety_file() if git_path_seq_dir() doesn't exist.

1. 452202c (sequencer: stop releasing the strbuf in
   write_message(), 2016-10-21)
2. f241ff0 (prepare the builtins for a libified merge_recursive(),
   2016-07-26)
3. 6e98de7 (sequencer (rebase -i): add support for the 'fixup' and
   'squash' commands, 2017-01-02)
4. https://lore.kernel.org/git/[email protected]/

Signed-off-by: Ævar Arnfjörð Bjarmason <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: avar

sequencer.c

@@ -2277,8 +2277,10 @@ static int do_pick_commit(struct repository *r,
 		reword = 1;
 	else if (is_fixup(command)) {
 		if (update_squash_messages(r, command, commit,
-					   opts, item->flags))
-			return -1;
+					   opts, item->flags)) {
+			res = -1;
+			goto leave;
+		}
 		flags |= AMEND_MSG;
 		if (!final_fixup)
 			msg_file = rebase_path_squash_msg();
@@ -2288,9 +2290,11 @@ static int do_pick_commit(struct repository *r,
 		} else {
 			const char *dest = git_path_squash_msg(r);
 			unlink(dest);
-			if (copy_file(dest, rebase_path_squash_msg(), 0666))
-				return error(_("could not rename '%s' to '%s'"),
-					     rebase_path_squash_msg(), dest);
+			if (copy_file(dest, rebase_path_squash_msg(), 0666)) {
+				res = error(_("could not rename '%s' to '%s'"),
+					    rebase_path_squash_msg(), dest);
+				goto leave;
+			}
 			unlink(git_path_merge_msg(r));
 			msg_file = dest;
 			flags |= EDIT_MSG;
@@ -2328,7 +2332,6 @@ static int do_pick_commit(struct repository *r,
 		free_commit_list(common);
 		free_commit_list(remotes);
 	}
-	strbuf_release(&msgbuf);
 
 	/*
 	 * If the merge was clean or if it failed due to conflict, we write
@@ -2402,6 +2405,7 @@ static int do_pick_commit(struct repository *r,
 leave:
 	free_message(commit, &msg);
 	free(author);
+	strbuf_release(&msgbuf);
 	update_abort_safety_file();
 
 	return res;