refdb_fs: refactor error handling in `refdb_reflog_fs__delete`

The function `refdb_reflog_fs__delete` uses the `if (!error && foobar())`
pattern of checking, where error conditions are being checked by following calls
to different code. This does not match our current style, where the call-site of
a function is usually directly responsible for checking the return value.

Convert the function to use `if ((error = foobar()) < 0) goto out;` style. Note
that this changes the code flow a bit: previously, we were always trying to
delete empty reference hierarchies even if deleting the reflog entry has failed.
This wasn't much of a problem -- if deletion failed, the hierarchy will still
contain at least one file and thus the function call was an expensive no-op.
Now, we will only perform this deletion if we have successfully removed the
reflog.

src/refdb_fs.c

@@ -2046,28 +2046,27 @@ cleanup:
 
 static int refdb_reflog_fs__delete(git_refdb_backend *_backend, const char *name)
 {
-	int error;
+	refdb_fs_backend *backend = (refdb_fs_backend *) _backend;
 	git_buf path = GIT_BUF_INIT;
-
-	git_repository *repo;
-	refdb_fs_backend *backend;
+	int error;
 
 	assert(_backend && name);
 
-	backend = (refdb_fs_backend *) _backend;
-	repo = backend->repo;
+	if ((error = retrieve_reflog_path(&path, backend->repo, name)) < 0)
+		goto out;
 
-	error = retrieve_reflog_path(&path, repo, name);
+	if (!git_path_exists(path.ptr))
+		goto out;
 
-	if (!error && git_path_exists(path.ptr)) {
-		error = p_unlink(path.ptr);
-		refdb_fs_backend__try_delete_empty_ref_hierarchie(backend, name, true);
-	}
+	if ((error = p_unlink(path.ptr)) < 0)
+		goto out;
 
+	refdb_fs_backend__try_delete_empty_ref_hierarchie(backend, name, true);
+
+out:
 	git_buf_dispose(&path);
 
 	return error;
-
 }
 
 int git_refdb_backend_fs(