localrepo: refactor filter computation

Separate loading part from access part to be able to tell with:
  repo._loadfilter(name)
  bool(name in repo.filterpats)
if there is a 'name' filter available.

mercurial/localrepo.py

@@ -510,7 +510,7 b' class localrepository(repo.repository):'
     def _link(self, f):
         return os.path.islink(self.wjoin(f))
 
-    def _filter(self, filter, filename, data):
+    def _loadfilter(self, filter):
         if filter not in self.filterpats:
             l = []
             for pat, cmd in self.ui.configitems(filter):
@@ -533,6 +533,9 b' class localrepository(repo.repository):'
                 l.append((mf, fn, params))
             self.filterpats[filter] = l
 
+    def _filter(self, filter, filename, data):
+        self._loadfilter(filter)
+
         for mf, fn, cmd in self.filterpats[filter]:
             if mf(filename):
                 self.ui.debug("filtering %s through %s\n" % (filename, cmd))