Do not prune after a backup that found a volume not mounted (closes #395)

snapshot create --prune purged the older snapshots holding a volume's
files when the volume, mounted below a configured path, was not
mounted: its empty mount point read as deleted files.

The scan now records in the local index each directory at or below a
configured path that is on a different device from its parent
directory. Under --prune, a run that does not find such a directory
recorded by an earlier snapshot of that name skips the prune, keeps
every older snapshot, names the directory and its snapshot, and exits
non-zero, as for an empty configured path. A volume retired on purpose
is cleared by removing the older snapshots with snapshot purge.

Model: opus-5-5

README.md

@@ -335,8 +335,16 @@ value where a list belongs or a word where a number belongs), is refused.
   either: every older snapshot is kept, and the command names the path and
   exits non-zero, on every run until the path holds something again. Take a
   path that is empty on purpose out of the config, or remove the older
-  snapshots by hand with `snapshot purge`. A file removed during the backup
-  does not stop the prune.
+  snapshots by hand with `snapshot purge`. When an earlier snapshot of the
+  same name in the local index found a volume mounted at or below a
+  configured path (a directory on a different filesystem from its parent)
+  and this backup finds none mounted there, the prune does not run either:
+  every older snapshot is kept, and the command names the directory and
+  exits non-zero, on every run until the volume is mounted again. The same
+  happens when that directory is no longer backed up, because it is excluded
+  or its configured path was removed. For a volume retired on purpose, remove
+  the older snapshots by hand with `snapshot purge`. A file removed during
+  the backup does not stop the prune.
 * `--keep-newer-than <duration>`: With `--prune`, keep snapshots newer than
   this duration instead of only the latest (e.g. `4w`, `30d`, `6mo`, `1y`).
   Without `--prune`, or with a value that is not a valid duration, the