Update the Git fetch API to be async

[zanieb]

Putting this up as a follow-up to https://github.com/astral-sh/uv/pull/6790

Drops the awkward nested tokio runtime in favor of making parts of the Git fetch API async. More work will be required to wrap some of the Git CLI calls in spawn_blocking to avoid blocking the runtime during slow subprocess invocations.

[charliermarsh]

I'm still in favor of this personally!

[zanieb]

Yeah we just need to make sure we're doing the right thing in all the subprocess calls — it will be a second before I prioritize this.