revlogutils: remember known metadata parents for issue6528

In the cases where the parent revs tell us for sure that the parent has
metadata, remember this fact to avoid content recomputations later.

mercurial/revlogutils/rewrite.py

@@ -653,6 +653,7 def _is_revision_affected_fast_inner(
 
     p1, p2 = parent_revs()
     if p1 == nullrev or p2 != nullrev:
+        metadata_cache[filerev] = True
         return False
 
     delta_parent = delta_base()