config: be strict on core.commentChar

We don't support comment _strings_ (at least not yet). And multi-byte
character encoding could also be misinterpreted.

The test with two commas is updated because it violates this. It's
added with the patch that introduces core.commentChar in eff80a9f
(Allow custom "comment char" - 2013-01-16). It's not clear to me _why_
that behavior is wanted.
Signed-off-by: NNguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

config.c

@@ -826,9 +826,13 @@ static int git_default_core_config(const char *var, const char *value)
 	if (!strcmp(var, "core.commentchar")) {
 		const char *comment;
 		int ret = git_config_string(&comment, var, value);
-		if (!ret)
+		if (ret)
+			return ret;
+		else if (comment[0] && !comment[1]) {
 			comment_line_char = comment[0];
-		return ret;
+		} else
+			return error("core.commentChar should only be one character");
+		return 0;
 	}
 
 	if (!strcmp(var, "core.askpass"))

t/t7508-status.sh

@@ -1350,8 +1350,7 @@ test_expect_success "status (core.commentchar with submodule summary)" '
 
 test_expect_success "status (core.commentchar with two chars with submodule summary)" '
 	test_config core.commentchar ";;" &&
-	git -c status.displayCommentPrefix=true status >output &&
-	test_i18ncmp expect output
+	test_must_fail git -c status.displayCommentPrefix=true status
 '
 
 test_expect_success "--ignore-submodules=all suppresses submodule summary" '