git config: report when trying to modify a non-existing repo config

It is a pilot error to call `git config section.key value` outside of
any Git worktree. The message

	error: could not lock config file .git/config: No such file or
	directory

is not very helpful in that situation, though. Let's print a helpful
message instead.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/config.c

@@ -352,6 +352,9 @@ static int get_colorbool(const char *var, int print)
 
 static void check_write(void)
 {
+	if (!given_config_source.file && !startup_info->have_repository)
+		die("not in a git directory");
+
 	if (given_config_source.use_stdin)
 		die("writing to stdin is not supported");
 

t/t1308-config-set.sh

@@ -218,4 +218,15 @@ test_expect_success 'check line errors for malformed values' '
 	test_i18ngrep "fatal: .*alias\.br.*\.git/config.*line 2" result
 '
 
+test_expect_success 'error on modifying repo config without repo' '
+	mkdir no-repo &&
+	(
+		GIT_CEILING_DIRECTORIES=$(pwd) &&
+		export GIT_CEILING_DIRECTORIES &&
+		cd no-repo &&
+		test_must_fail git config a.b c 2>err &&
+		grep "not in a git directory" err
+	)
+'
+
 test_done