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

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.

--- a/mercurial/context.py	Fri Aug 01 22:05:16 2014 -0700
+++ b/mercurial/context.py	Fri Aug 01 22:07:29 2014 -0700
@@ -598,6 +598,9 @@
                 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 @@
         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]):