filectx: refactor filectx.rev() to use filectx._changeid

The code in filectx.rev() was identical to filectx._changeid. Fixing this
allows alternative filectx implementations to only override _changeid.

mercurial/context.py

@@ -503,14 +503,8 b' class filectx(object):'
         return self._changectx.flags(self._path)
     def filelog(self):
         return self._filelog
-
     def rev(self):
-        if '_changectx' in self.__dict__:
-            return self._changectx.rev()
-        if '_changeid' in self.__dict__:
-            return self._changectx.rev()
-        return self._filelog.linkrev(self._filerev)
-
+        return self._changeid
     def linkrev(self):
         return self._filelog.linkrev(self._filerev)
     def node(self):