Merge branch 'js/config-set-in-non-repository'

"git config section.var value" to set a value in per-repository
configuration file failed when it was run outside any repository,
but didn't say the reason correctly.

* js/config-set-in-non-repository:
  git config: report when trying to modify a non-existing repo config

builtin/config.c

@@ -377,6 +377,9 @@ 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");