Replace libgit2 with gitoxide

[o2sh]

Following #621, we wish to drop libgit2 and use gitoxide instead.

What's left:

• status index/worktree (without diffstats, and submodule support with 'is-dirty' granularity)
• status untracked + recurse untracked dirs
• write index back (even though we shouldn't do it unless it's lossless, should be discussed at least)

[Byron]

Also see the corresponding tracking ticket at gitoxide, showing which features are implemented already.

@o2sh Would you be OK if I contributed the most promising part related to traversing the commit-graph for a start? This would mean that build times increase due to building git2 and gitoxide (minimal, only the required crates at first). Otherwise we'd have to wait until all features are available, which is probably no more than a couple of months.

That said, I'd prefer to get started right away and am happy to see the transition through until completion.

[o2sh]

Sure, the commit-graph traversal is indeed the most crucial part.
Looking forward to see your contribution.

[o2sh]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs.

[Byron]

git status is going to happen soon and then the issue can be properly resolved.