Start on a Git-based review flow (#27103)

Release Notes:

- N/A

---------

Co-authored-by: Nathan Sobo <nathan@zed.dev>

crates/fs/src/fake_git_repo.rs

@@ -408,4 +408,12 @@ impl GitRepository for FakeGitRepository {
     ) -> BoxFuture<Result<String>> {
         unimplemented!()
     }
+
+    fn checkpoint(&self, _cx: AsyncApp) -> BoxFuture<Result<git::Oid>> {
+        unimplemented!()
+    }
+
+    fn restore_checkpoint(&self, _oid: git::Oid, _cx: AsyncApp) -> BoxFuture<Result<()>> {
+        unimplemented!()
+    }
 }