Avoid panic when re-locking with precise commit (#5863)

## Summary

Very subtle bug. The scenario is as follows:

- We resolve: `elmer-circuitbuilder = { git =
"https://github.com/ElmerCSC/elmer_circuitbuilder.git" }`

- The user then changes the request to: `elmer-circuitbuilder = { git =
"https://github.com/ElmerCSC/elmer_circuitbuilder.git", rev =
"44d2f4b19d6837ea990c16f494bdf7543d57483d" }`

- When we go to re-lock, we note two facts:

1. The "default branch" resolves to
`44d2f4b19d6837ea990c16f494bdf7543d57483d`.
2. The metadata for `44d2f4b19d6837ea990c16f494bdf7543d57483d` is
(whatever we grab from the lockfile).

- In the resolver, we then ask for the metadata for
`44d2f4b19d6837ea990c16f494bdf7543d57483d`. It's already in the cache,
so we return it; thus, we never add the
`44d2f4b19d6837ea990c16f494bdf7543d57483d` ->
`44d2f4b19d6837ea990c16f494bdf7543d57483d` mapping to the Git resolver,
because we never have to resolve it.

This would apply for any case in which a requested tag or branch was
replaced by its precise SHA. Replacing with a different commit is fine.

It only applied to `tool.uv.sources`, and not PEP 508 URLs, because the
underlying issue is that we aren't consistent about "automatically"
extracting the precise commit from a Git reference.

Closes https://github.com/astral-sh/uv/issues/5860.

crates/uv-requirements/src/lookahead.rs

@@ -261,10 +261,11 @@ fn required_dist(requirement: &Requirement) -> Result<Option<Dist>, distribution
             subdirectory,
             url,
         } => {
-            let mut git_url = GitUrl::new(repository.clone(), reference.clone());
-            if let Some(precise) = precise {
-                git_url = git_url.with_precise(*precise);
-            }
+            let git_url = if let Some(precise) = precise {
+                GitUrl::from_commit(repository.clone(), reference.clone(), *precise)
+            } else {
+                GitUrl::from_reference(repository.clone(), reference.clone())
+            };
             Dist::Source(SourceDist::Git(GitSourceDist {
                 name: requirement.name.clone(),
                 git: Box::new(git_url),