Merge branch 'ap/merge-stop-at-prepare-commit-msg-failure'

"git merge" started calling prepare-commit-msg hook like "git
commit" does some time ago, but forgot to pay attention to the exit
status of the hook.  t7505 may want a general clean-up but that is
a different topic.

* ap/merge-stop-at-prepare-commit-msg-failure:
  merge: Honor prepare-commit-msg return code

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);

t/t7505-prepare-commit-msg-hook.sh

@@ -167,5 +167,19 @@ 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 &&
+	rm -f "$HOOK" &&
+	git commit -m other &&
+	write_script "$HOOK" <<-EOF
+	exit 1
+	EOF
+	git checkout - &&
+	test_must_fail git merge other
+
+'
 
 test_done