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-git/src/git.rs

@@ -3,8 +3,10 @@
 //! Source: <https://github.com/rust-lang/cargo/blob/23eb492cf920ce051abfc56bbaf838514dc8365c/src/cargo/sources/git/utils.rs>
 use std::fmt::Display;
 use std::path::{Path, PathBuf};
-use std::str::{self};
+use std::str::{self, FromStr};
 
+use crate::sha::GitOid;
+use crate::GitSha;
 use anyhow::{anyhow, Context, Result};
 use cargo_util::{paths, ProcessBuilder};
 use reqwest::StatusCode;
@@ -13,8 +15,6 @@ use tracing::debug;
 use url::Url;
 use uv_fs::Simplified;
 
-use crate::sha::GitOid;
-
 /// A file indicates that if present, `git reset` has been done and a repo
 /// checkout is ready to go. See [`GitCheckout::reset`] for why we need this.
 const CHECKOUT_READY_LOCK: &str = ".ok";
@@ -108,6 +108,15 @@ impl GitReference {
             Self::DefaultBranch => "default branch",
         }
     }
+
+    /// Returns the precise [`GitSha`] of this reference, if it's a full commit.
+    pub(crate) fn as_sha(&self) -> Option<GitSha> {
+        if let Self::FullCommit(rev) = self {
+            Some(GitSha::from_str(rev).expect("Full commit should be exactly 40 characters"))
+        } else {
+            None
+        }
+    }
 }
 
 impl Display for GitReference {