filemerge: fix a missing attribute usage

Flagged by both pytype and VSCode.

Differential Revision: https://phab.mercurial-scm.org/D7465

mercurial/filemerge.py

@@ -119,7 +119,7 b' class absentfilectx(object):'
         """
         return not (
             fctx.isabsent()
-            and fctx.ctx() == self.ctx()
+            and fctx.changectx() == self.changectx()
             and fctx.path() == self.path()
         )