git config: error when editing a repo config and not being in one

Let's throw an error on this specific case. If the user specifies the
config file, he must know what he is doing.

Teemu Likonen pointed this out.

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-config.c

@@ -390,6 +390,8 @@ int cmd_config(int argc, const char **argv, const char *unused_prefix)
 	}
 	else if (actions == ACTION_EDIT) {
 		check_argc(argc, 0, 0);
+		if (!config_exclusive_filename && nongit)
+			die("not in a git directory");
 		git_config(git_default_config, NULL);
 		launch_editor(config_exclusive_filename ?
 			      config_exclusive_filename : git_path("config"),