Merge branch 'ad/commit-have-m-option'

"git commit" misbehaved in a few minor ways when an empty message
is given via -m '', all of which has been corrected.

* ad/commit-have-m-option:
  commit: do not ignore an empty message given by -m ''
  commit: --amend -m '' silently fails to wipe message

builtin/commit.c

@@ -695,7 +695,7 @@ static int prepare_to_commit(const char *index_file, const char *prefix,
 		}
 	}
 
-	if (message.len) {
+	if (have_option_m) {
 		strbuf_addbuf(&sb, &message);
 		hook_arg1 = "message";
 	} else if (logfile && !strcmp(logfile, "-")) {
@@ -1172,9 +1172,9 @@ static int parse_and_validate_options(int argc, const char *argv[],
 		f++;
 	if (f > 1)
 		die(_("Only one of -c/-C/-F/--fixup can be used."));
-	if (message.len && f > 0)
+	if (have_option_m && f > 0)
 		die((_("Option -m cannot be combined with -c/-C/-F/--fixup.")));
-	if (f || message.len)
+	if (f || have_option_m)
 		template_file = NULL;
 	if (edit_message)
 		use_message = edit_message;

t/t7501-commit.sh

@@ -200,6 +200,26 @@ test_expect_success '--amend --edit of empty message' '
 	test_cmp expect msg
 '
 
+test_expect_success '--amend to set message to empty' '
+	echo bata >file &&
+	git add file &&
+	git commit -m "unamended" &&
+	git commit --amend --allow-empty-message -m "" &&
+	git diff-tree -s --format=%s HEAD >msg &&
+	echo "" >expect &&
+	test_cmp expect msg
+'
+
+test_expect_success '--amend to set empty message needs --allow-empty-message' '
+	echo conga >file &&
+	git add file &&
+	git commit -m "unamended" &&
+	test_must_fail git commit --amend -m "" &&
+	git diff-tree -s --format=%s HEAD >msg &&
+	echo "unamended" >expect &&
+	test_cmp expect msg
+'
+
 test_expect_success '-m --edit' '
 	echo amended >expect &&
 	git commit --allow-empty -m buffer &&