match: make matchfn a method on the class

This makes it easier to override in subclasses, so they don't have to
assign the attribute with a lambda.

mercurial/match.py

@@ -209,7 +209,6 b' class basematcher(object):'
         self._cwd = cwd
         if badfn is not None:
             self.bad = badfn
-        self.matchfn = lambda f: False
 
     def __call__(self, fn):
         return self.matchfn(fn)
@@ -266,6 +265,9 b' class basematcher(object):'
         '''Returns True if f is in .files().'''
         return f in self._fileset
 
+    def matchfn(self, f):
+        return False
+
     def visitdir(self, dir):
         '''Decides whether a directory should be visited based on whether it
         has potential matches in it or one of its subdirectories. This is