Bugfix: Update `mount` latest symlink after snapshot reload

When `restic mount` was kept running while new snapshots were
created, the new snapshots appeared in the mountpoint, but the `latest`
symlink could still point to the previously latest snapshot. Restic now
invalidates the cached snapshot directory entries after a snapshot reload so
that `latest` points to the newest snapshot.

https://github.com/restic/restic/issues/5722
https://github.com/restic/restic/pull/21873
