Prevent gsb backup from overwriting existing tags

[OpenBagTwo]

Summary

Fixes #32

List of Changes

• gsb backup --tag will now fail if the current HEAD is already a tagged backup (meaning annotated tag), e.g.

  The current HEAD is already tagged as gsb2023.10.11+125502

• gsb rewind will also not create an unnecessary tag if the current state is already tagged
  • as would result in running two rewinds back-to-back, a la https://github.com/OpenBagTwo/gsb/blob/42d2ed6ff927771f4a89f6f660f37f17f7533ea8/gsb/test/test_rewind.py#L89-L101
• This PR also includes some fixes stemming from the gsb test dogfooding
• I also performed some mypy and CLI help string nits

Tech Debt and Other Concerns

Validation Performed

That quote above is from me running gsb backup --tag twice in a row on my own GSB-managed save repo

PR Type

• [ ] This PR introduces a breaking change (will require a bump in the minor version)
• [ ] The changes in this PR are high urgency and necessitate a hotfix or patch release (will require rebasing off of release)
• [ ] This is a release (staging) PR (maintainer use only)

Checklist:

• [x] I have read the contributor's guide
• [x] I have run mkdocs serve locally and ensured that all API docs and changes I have made to the static pages are rendering correctly, with all links working
• [x] All tech debt concerns have been resolved, documented as issues, or otherwise accepted
• [x] I agree to license my contribution to this project under the GNU Public License v3

Note

This PR marks the feature cutoff for v0.0.3 (#36 / #40 is out of my hands and thus will likely get bumped to the next release). After this PR is merged, I'll be creating a release candidate and opening a staging branch which will include any additional documentation changes (read: #38) or bugfixes that get surfaced during release testing