Backup cleaner

[OpenBagTwo]

GIVEN Steve ran gsb list --verbose (#7) and noticed that his managed savegame my-savegame is taking up a ton of space

WHEN he runs the command gsb clean my-savegame

THEN gsb takes him through a series of interactive prompts that give him options for reducing the repo size, reporting after each operation how much space that operation saved* after it's completed

SO that Steve is able to keep his backups clean and organized

Notes

• *I don't know if there's any way for git to tell you in advance how much an operation will free up.
• Operations to perform:
  • git gc
  • If we're going with the tag approach, I guess this is where we'd go through and squash all the intermediate commits into the tags
  • Stealing leveraging some of the awesomeness of BFG is definitely on the table ("hey, did you actually want to exclude this massive do-nothing file?")
  • An interactive backup deleter (see: #8) ("Select the backups to delete by hash, index, or literally by checking them off one by one")
• Steve should also be able to specify gsb clean --all which may only offer prompts for the no-regrets ops that can be applied uniformly to all repos
  • gsb clean --all is a perfect candidate for daemonization, but that's out of scope.
• when performing any rebasing or squashing operation, it's vitally important that timestamps be preserved (see #17) to correspond with the actual timestamp of the preserved backups (that is squashing A->B->C->D' into A->C', C' should have a commit date matching C. Further reading.

[OpenBagTwo]

It's worth noting that this functionality is not available in the underlying library: https://github.com/libgit2/libgit2/issues/3247

so it would need to rely on the standalone git executable being available.