refdb: remove a check-delete race when removing a loose ref

It does not help us to check whether the file exists before trying to
unlink it since it might be gone by the time unlink is called.

Instead try to remove it and handle the resulting error if it did not
exist.

src/refdb_fs.c

@@ -1281,15 +1281,14 @@ static int refdb_fs_backend__delete_tail(
 	if (git_buf_joinpath(&loose_path, backend->path, ref_name) < 0)
 		return -1;
 
-	if (git_path_isfile(loose_path.ptr)) {
-		error = p_unlink(loose_path.ptr);
-		loose_deleted = 1;
-	}
-
-	git_buf_free(&loose_path);
 
-	if (error != 0)
+	error = p_unlink(loose_path.ptr);
+	if (error < 0 && errno == ENOENT)
+		error = 0;
+	else if (error < 0)
 		goto cleanup;
+	else if (error == 0)
+		loose_deleted = 1;
 
 	if ((error = packed_reload(backend)) < 0)
 		goto cleanup;
@@ -1312,6 +1311,7 @@ static int refdb_fs_backend__delete_tail(
 	error = packed_write(backend);
 
 cleanup:
+	git_buf_free(&loose_path);
 	git_filebuf_cleanup(file);
 
 	return error;