git: actually copy treemanifest instances in .copy() (issue6398)

The implementation here is so simple I honestly have no idea why I
didn't do it at the time. Hopefully there's not some nuance past-me
forgot to write down.

Differential Revision: https://phab.mercurial-scm.org/D8993

hgext/git/manifest.py

@@ -217,7 +217,9 b' class gittreemanifest(object):'
             return b''
 
     def copy(self):
-        pass
+        return gittreemanifest(
+            self._git_repo, self._tree, dict(self._pending_changes)
+        )
 
     def items(self):
         for f in self: