[PATCH] merge: Honor prepare-commit-msg return code

prepare-commit-msg hook is run when committing to prepare the log
message. If the exit-status is non-zero, the commit should be aborted.

While the script is run before committing a successful merge, the
exit-status is ignored and a non-zero exit doesn't abort the commit.

Abort the commit if prepare-commit-msg returns with a non-zero status
when committing a successful merge.

Signed-off-by: Antoine Pelisse <redacted>
---
 builtin/merge.c                    |  5 +++--
 t/t7505-prepare-commit-msg-hook.sh | 13 +++++++++++++
 2 files changed, 16 insertions(+), 2 deletions(-)

diff --git a/builtin/merge.c b/builtin/merge.c
index a96e8ea..3a31c4b 100644
--- a/builtin/merge.c
+++ b/builtin/merge.c

@@ -800,8 +800,9 @@ static void prepare_to_commit(struct commit_list *remoteheads)
        if (0 < option_edit)
                strbuf_add_lines(&msg, "# ", comment, strlen(comment));
        write_merge_msg(&msg);
-       run_hook(get_index_file(), "prepare-commit-msg",
-                git_path("MERGE_MSG"), "merge", NULL, NULL);
+       if (run_hook(get_index_file(), "prepare-commit-msg",
+                    git_path("MERGE_MSG"), "merge", NULL, NULL))
+               abort_commit(remoteheads, NULL);
        if (0 < option_edit) {
                if (launch_editor(git_path("MERGE_MSG"), NULL, NULL))
                        abort_commit(remoteheads, NULL);

diff --git a/t/t7505-prepare-commit-msg-hook.sh b/t/t7505-prepare-commit-msg-hook.sh
index 5b4b694..bc497bc 100755
--- a/t/t7505-prepare-commit-msg-hook.sh
+++ b/t/t7505-prepare-commit-msg-hook.sh

@@ -167,5 +167,18 @@ test_expect_success 'with failing hook (--no-verify)' '

 '

+test_expect_success 'with failing hook (merge)' '
+
+       git checkout -B other HEAD@{1} &&
+       echo "more" >> file &&
+       git add file &&
+       chmod -x $HOOK &&
+       git commit -m other &&
+       chmod +x $HOOK &&
+       git checkout - &&
+       head=`git rev-parse HEAD` &&