radicle-surf: consolidate `Commit` types

[FintanH]

Currently, we have 3 types of Commits that are being worked with in radicle-surf:

• git2::Commit
• radicle_surf::git::Commit
• radicle_surf::Commit

I think this can result in confusing semantics within some of the code. For example, I noticed in a few places that we may have a git::Commit just to go fetch a git2::Commit.

I think we should: a) Try to consolidate the two surf-defined Commits into a single type if possible b) Audit the code for this kind of extra trips to the repository and clean them up

[keepsimple1]

I've created a PR to consolidate object::Commit and Commit. The git2::Commit is already hidden from the public API (if leaked, would be a bug).

[keepsimple1]

done.