improve error message in config.c

Show errno if opening a lockfile fails.

Signed-off-by: Alex Riesen <raa.lkml@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

config.c

@@ -1001,7 +1001,7 @@ int git_config_set_multivar(const char* key, const char* value,
 	lock = xcalloc(sizeof(struct lock_file), 1);
 	fd = hold_lock_file_for_update(lock, config_filename, 0);
 	if (fd < 0) {
-		error("could not lock config file %s", config_filename);
+		error("could not lock config file %s: %s", config_filename, strerror(errno));
 		free(store.key);
 		ret = -1;
 		goto out_free;