localrepo: use self.wvfs.islink directly

We are about to deprecate the helper function.

mercurial/localrepo.py

@@ -1028,7 +1028,7 b' class localrepository(object):'
         self._datafilters[name] = filter
 
     def wread(self, filename):
-        if self._link(filename):
+        if self.wvfs.islink(filename):
             data = self.wvfs.readlink(filename)
         else:
             data = self.wvfs.read(filename)