Avoid querying GitHub on repeated install invocations

[charliermarsh]

Summary

If you run cargo run pip install "pip-test-package @ git+https://github.com/pypa/pip-test-package@5547fa909e83df8bd743d3978d6667497983a4b7" repeatedly, then every time, we'll take the "GitHub fast path" every time, even if the package is already cached. This PR adds logic to skip the fast path if the reference looks like a commit that we've already checked out.

Closes #12760.

[charliermarsh]

I think there's a better fix here, which is: we move all this metadata querying into uv-git. So we have a method like fetch_metadata that looks like fetch, but is "allowed" to short-circuit by getting the metadata. It's much more involved, though.