Bugfix: Prevent mounting over the repository directory

Using a local repository directory as the `mount` target — or a path
that contains it, or that it contains — caused the FUSE server to
read its own backend files through the new mount, deadlocking the
kernel and requiring a long reboot to recover.

Restic now resolves both paths and refuses any such overlap with a
clear error before mounting.

https://github.com/restic/restic/issues/5234
https://github.com/restic/restic/pull/5348
