send/receive fails if snapshot_dir is a mount point

Btrbk fails if the directory pointed by `/` is a mount point.

It is not uncommon to have snapshot directory on a mointpoint, the Arch Wiki even suggests such a setup: https://wiki.archlinux.org/index.php/Snapper#Suggested_filesystem_layout

As this introduces quite some architectural changes in btrbk, a fix for this is currently living on a feature-branch: [snapdir-mountpoint](/digint/btrbk/tree/snapdir-mountpoint).

Review the snapdir-mountpoint feature branch and trace the send/receive behavior for a snapshot directory that is itself a mount point. Confirm the intended behavior against the issue description and Arch Wiki setup, then verify that snapshot send and receive work correctly with this layout.

Written by the indexing model from the issue text.