config_file: do not refresh read-only backends

If calling `config_refresh` on a read-only configuration file backend,
then we will segfault when comparing the timestamp of the file due to
`path` being uninitialized. As a read-only snapshot should not be
refreshed anyway and stay consistent, we can simply return early when
calling `config_refresh` on a read-only snapshot.

src/config_file.c

@@ -231,8 +231,10 @@ static int config_refresh(git_config_backend *cfg)
 	git_config_entries *entries = NULL;
 	int error, modified;
 
-	error = config_is_modified(&modified, &b->file);
-	if (error < 0 && error != GIT_ENOTFOUND)
+	if (cfg->readonly)
+		return 0;
+
+	if ((error = config_is_modified(&modified, &b->file)) < 0 && error != GIT_ENOTFOUND)
 		goto out;
 
 	if (!modified)