context: add a method to efficiently filter by match if possible

For non-working contexts, walk and matches do the same thing. For working
contexts, walk stats all the files and looks for unknown files, while matches
just filters the dirstate by match.

mercurial/context.py

@@ -598,6 +598,9 b' class changectx(basectx):'
                 continue
             match.bad(fn, _('no such file in rev %s') % self)
 
+    def matches(self, match):
+        return self.walk(match)
+
 class basefilectx(object):
     """A filecontext object represents the common logic for its children:
     filectx: read-only access to a filerevision that is already present
@@ -1144,6 +1147,9 b' class committablectx(basectx):'
         return sorted(self._repo.dirstate.walk(match, sorted(self.substate),
                                                True, False))
 
+    def matches(self, match):
+        return sorted(self._repo.dirstate.matches(match))
+
     def ancestors(self):
         for a in self._repo.changelog.ancestors(
             [p.rev() for p in self._parents]):