localrepo: Unlock symbolic refs when the update fails

We use a safe.LockingFileWriter to get transactional semantics when updating symbolic references. This mechanism will create a lock for the file that is about to be updated, votes on the contents of the file and then finally unlocks it if everything went alright. In case something goes wrong though the caller must make sure to close the writer, or otherwise we may leak the lockfile and thus also obstruct all subsequent writes to that symbolic reference. We don't do this correctly though.

Fix this bug by always unlocking the locking file writer.

Fixes #4075 (closed)