committablefilectx: override linkrev() to point to the associated changectx

This is necessary to annotate workingctx revision. basefilectx.linkrev() can't
be used because committablefilectx has no filelog.

committablefilectx looks for parents() from self._changectx. That means fctx
is linked to self._changectx, so linkrev() can simply be aliased to rev().

mercurial/context.py

@@ -1594,6 +1594,10 b' class committablefilectx(basefilectx):'
     def __nonzero__(self):
         return True
 
+    def linkrev(self):
+        # linked to self._changectx no matter if file is modified or not
+        return self.rev()
+
     def parents(self):
         '''return parent filectxs, following copies if necessary'''
         def filenode(ctx, path):