match: define exactmatcher.matchfn statically

This should eliminate the reference cycle, self.matchfn -> self.exact -> self.

mercurial/match.py

@@ -415,7 +415,8 class exactmatcher(basematcher):
             self._files = files
         else:
             self._files = list(files)
-        self.matchfn = self.exact
+
+    matchfn = basematcher.exact
 
     @propertycache
     def _dirs(self):