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.

crates/uv-git/src/lib.rs

@@ -47,6 +47,11 @@ impl GitUrl {
         }
     }
 
+    /// Returns `true` if the reference is a full commit.
+    pub fn is_full_commit(&self) -> bool {
+        matches!(self.reference, GitReference::FullCommit(_))
+    }
+
     /// Return the precise commit, if known.
     pub fn precise(&self) -> Option<GitSha> {
         self.precise