changectx: use unfiltered changelog to access parents of unfiltered revs

If a revision is not filtered, we know that its parents are not either. So we
can take a shortcut. This shortcut avoid the computation of all filtered revs in
some cases.

Differential Revision: https://phab.mercurial-scm.org/D7487

mercurial/context.py

@@ -523,7 +523,12 b' class changectx(basectx):'
     @propertycache
     def _parents(self):
         repo = self._repo
-        p1, p2 = repo.changelog.parentrevs(self._rev)
+        if self._maybe_filtered:
+            cl = repo.changelog
+        else:
+            cl = repo.unfiltered().changelog
+
+        p1, p2 = cl.parentrevs(self._rev)
         if p2 == nullrev:
             return [repo[p1]]
         return [repo[p1], repo[p2]]

tests/test-repo-filters-tiptoe.t

@@ -58,7 +58,6 b' Getting basic changeset inforation about'
 Getting status of null
 
   $ hg status --change null
-  debug.filters: computing revision filter for "visible"
 
 Getting status of working copy