gsb backup should allow you to combine new changes with previous backups

[OpenBagTwo]

GIVEN Alex is managing his game save with gsb

WHEN she creates a new backup (tagged or untagged) via the command: gsb backup -c [--tag <message>]

THEN the new backup should be combined with / overwrite the previous backup if that backup is untagged

AND

WHEN she creates a new backup (tagged or untagged) via the command: gsb backup -cc [--tag <message>] THEN the new backup should be combined with / overwrite / replace all backups since the last tagged backup

SO that her backup history isn't cluttered by a bunch of backups that were too unimportant to tag

Notes

While this might seem like a git commit --amend it's actually better implemented as

git reset --soft <last-backup-to-keep>
git add <stuff>
git commit