Threadability

[FintanH]

Threading is a sore point. Send and Sync are indications of structures being able to be passed around in async code. The issue is that git2::Repository is not able to be threaded (probably for a good reason to be fair).

We should investigate if we can ease the pain here and see if we can use some methods to make radicle-surf usable with async code. It only reads the Repository data and does no mutation, we theoretically should be fine.