match: make basematcher._files a @propertycache

This will make it easier to override in subclasses (otherwise the
function @propertycache object will be replaced by the
super-constructor call)..

mercurial/match.py

@@ -209,7 +209,6 b' class basematcher(object):'
         self._cwd = cwd
         if badfn is not None:
             self.bad = badfn
-        self._files = [] # exact files and roots of patterns
         self.matchfn = lambda f: False
 
     def __call__(self, fn):
@@ -247,6 +246,10 b' class basematcher(object):'
         Otherwise it is relative to the root of the repo.'''
         return self.rel(f)
 
+    @propertycache
+    def _files(self):
+        return []
+
     def files(self):
         '''Explicitly listed files or patterns or roots:
         if no patterns or .always(): empty list,