Enhancement: Support `--group-by` for backup parent selection

The backup command by default selected the parent snapshot based on the hostname
and the backup targets. When the backup path list changed, the backup command
was unable to determine a suitable parent snapshot and had to read all
files again.

The new `--group-by` option for the backup command allows filtering snapshots
for the parent selection by `host`, `paths` and `tags`. It defaults to
`host,paths` which selects the latest snapshot with hostname and paths matching
those of the backup run. It should be used consistently with `forget --group-by`.

https://github.com/restic/restic/issues/3941
https://github.com/restic/restic/pull/4081