filectx: allow custom comparators

We're going to introduce other sorts of filectxes very soon, and we'd like the
cmp method to function properly (i.e. commutatively) for them. The only way to
make that happen is for this cmp method to call into that specialized one if
that defines a custom comparator.

mercurial/context.py

@@ -747,11 +747,15 class basefilectx(object):
     def islink(self):
         return 'l' in self.flags()
 
+    _customcmp = False
     def cmp(self, fctx):
         """compare with other file context
 
         returns True if different than fctx.
         """
+        if fctx._customcmp:
+            return fctx.cmp(self)
+
         if (fctx._filerev is None
             and (self._repo._encodefilterpats
                  # if file data starts with '\1\n', empty metadata block is