Repo onboarding proof-of-concept

[OpenBagTwo]

Summary

Addresses #1, save for the CLI, and resolves #15 by proceeding forward using pygit2 as a backend instead of GitPython

List of Changes

• Defines a save-specific specification file: .gsb_manifest
• Implements onboard.create_repo to the specification of #1. Explicitly: calling that method will:
  1. Initialize (or re-initialize) a repository in the specified location
  2. Create a configuration with a manifest of the files to be tracked
  3. Add all requested files along with the gsb cruft and create an initial commit in a way that is transparently (via the "committer" field) tied to gsb
  4. Create a unique annotated tag to mark the start of gsb tracking
• Additionally:
  • If there's already a .gsb_manifest in the directory, the method will raise an error
  • If there's already a Git repo in the directory, this method will onboard it just fine, appending onto the existing history
  • The user is also able to specify a list of patterns to append to .gitignore
• Abstracts away all the calls to pygit2 into a module called _git so that if I decide later to switch to another library, I only have to do it in one place

Tech Debt and Other Concerns

pygit2 is extremely low-level, the documentation is lacking in certain places, and there are definitely some limitations to what I can do / what I can do easily. That being said nothing so far has risen to the level of "tech debt," and I do think it'll be able to handle everything I need it to do. Plus, it's actively maintained!

Validation Performed

• [x] Comprehensive unit tests (100% coverage, fwiw)
• [x] I've been running a lot of the lower-level _git utilities on some existing repos to tweak their behavior, and I'm confident they're behaving as intended

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

[OpenBagTwo]

Very interesting that the libgit2.so (on the Ubuntu worker at least) is old enough that the default branch is still master...