project: reuse loaded refs when resolving HEAD

When the regular checkout has already loaded GitRefs, derive its current
branch and HEAD object from that snapshot instead of spawning
symbolic-ref or rev-parse again. Keep linked Git worktrees on their
worktree-specific HEAD path because the shared repository HEAD is not
authoritative for them.

Bug: 543851900
Bug: 553599402
Change-Id: Iec456be5f3c279865b4165a75d1c6df36e331dbc
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/623181
Commit-Queue: Gavin Mak <gavinmak@google.com>
Reviewed-by: Brian Gan <brgan@google.com>
Tested-by: Gavin Mak <gavinmak@google.com>

git_refs.py

@@ -48,6 +48,11 @@ class GitRefs:
         self._EnsureLoaded()
         return self._symref.get(HEAD) or self._phyref.get(HEAD, "")
 
+    @property
+    def is_loaded(self) -> bool:
+        """Whether a ref snapshot has already been loaded."""
+        return self._phyref is not None
+
     def get(self, name):
         try:
             return self.all[name]