Allow conflicting Git URLs that refer to the same commit SHA (#2769)
## Summary
This PR leverages our lookahead direct URL resolution to significantly
improve the range of Git URLs that we can accept (e.g., if a user
provides the same requirement, once as a direct dependency, and once as
a tag). We did some of this in #2285, but the solution here is more
general and works for arbitrary transitive URLs.
Closes https://github.com/astral-sh/uv/issues/2614.
