Merge branch 'jk/commit-C-pick-empty'

"git commit --allow-empty-message -C $commit" did not work when the
commit did not have any log message.

* jk/commit-C-pick-empty:
  commit: do not complain of empty messages from -C

builtin/commit.c

@@ -676,9 +676,8 @@ static int prepare_to_commit(const char *index_file, const char *prefix,
 	} else if (use_message) {
 		char *buffer;
 		buffer = strstr(use_message_buffer, "\n\n");
-		if (!use_editor && (!buffer || buffer[2] == '\0'))
-			die(_("commit has empty message"));
-		strbuf_add(&sb, buffer + 2, strlen(buffer + 2));
+		if (buffer)
+			strbuf_add(&sb, buffer + 2, strlen(buffer + 2));
 		hook_arg1 = "commit";
 		hook_arg2 = use_message;
 	} else if (fixup_message) {

t/t7500-commit.sh

@@ -223,7 +223,8 @@ test_expect_success 'Commit without message is allowed with --allow-empty-messag
 	git add foo &&
 	>empty &&
 	git commit --allow-empty-message <empty &&
-	commit_msg_is ""
+	commit_msg_is "" &&
+	git tag empty-message-commit
 '
 
 test_expect_success 'Commit without message is no-no without --allow-empty-message' '
@@ -240,6 +241,14 @@ test_expect_success 'Commit a message with --allow-empty-message' '
 	commit_msg_is "hello there"
 '
 
+test_expect_success 'commit -C empty respects --allow-empty-message' '
+	echo more >>foo &&
+	git add foo &&
+	test_must_fail git commit -C empty-message-commit &&
+	git commit -C empty-message-commit --allow-empty-message &&
+	commit_msg_is ""
+'
+
 commit_for_rebase_autosquash_setup () {
 	echo "first content line" >>foo &&
 	git add foo &&