`--keep-tag invalid-tag` was previously able to wipe all snapshots in a
repository. As a user specified a `--keep-*` option this is likely
unintentional. This forbid deleting all snapshot if a `--keep-*` option
was specified to prevent data loss. (Not specifying such an option
currently also causes the command to abort)
This commit is a followup to the addition of the --group-by flag for the
snapshots command. Adding the grouping code there introduced duplicated
code (the forget command also does grouping). This commit refactors
boths sides to only use shared code.