localrepo: optimize internode status calls using match.always

Introduce match.always() to check if a match object always says yes, i.e.
None was passed in. If so, mfmatches should not bother iterating every file in
the repository.

mercurial/localrepo.py

@@ -1333,6 +1333,8 b' class localrepository(repo.repository):'
 
         def mfmatches(ctx):
             mf = ctx.manifest().copy()
+            if match.always():
+                return mf
             for fn in mf.keys():
                 if not match(fn):
                     del mf[fn]

mercurial/match.py

@@ -118,6 +118,8 b' class match(object):'
         return self._files
     def anypats(self):
         return self._anypats
+    def always(self):
+        return False
 
 class exact(match):
     def __init__(self, root, cwd, files):
@@ -126,6 +128,8 b' class exact(match):'
 class always(match):
     def __init__(self, root, cwd):
         match.__init__(self, root, cwd, [])
+    def always(self):
+        return True
 
 class narrowmatcher(match):
     """Adapt a matcher to work on a subdirectory only.