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

Previously, the `backup` command by default selected the parent snapshot based
on the hostname and the backup paths. 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. This matches the behavior of prior restic versions.

The new `--group-by` option should be set to the same value as passed to
`forget --group-by`.

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