git: add readfast() method to manifest

Mysteriously, on a different system a handful of things don't work
without this, including log. This is especially odd to be because log
works in the test. In any event, it's legal for readfast() to just
return read(), so that's what we do.

Differential Revision: https://phab.mercurial-scm.org/D8262

hgext/git/manifest.py

@@ -195,6 +195,9 b' class gittreemanifestctx(object):'
     def read(self):
         return gittreemanifest(self._repo, self._tree, None)
 
+    def readfast(self, shallow=False):
+        return self.read()
+
     def copy(self):
         # NB: it's important that we return a memgittreemanifestctx
         # because the caller expects a mutable manifest.