filectx: fix annotate to not directly instantiate filectx

28a302e9225d changed basefilectx.annotate() to directly instantiate new
filectx's instead of going through self.filectx(), this breaks extensions that
replace the filectx class, and would also break future uses that would need
memfilectx's.

mercurial/context.py

@@ -893,8 +893,7 b' class basefilectx(object):'
         base = self
         introrev = self.introrev()
         if self.rev() != introrev:
-            base = filectx(self._repo, self._path, filelog=self.filelog(),
-                           fileid=self.filenode(), changeid=introrev)
+            base = self.filectx(self.filenode(), changeid=introrev)
 
         # This algorithm would prefer to be recursive, but Python is a
         # bit recursion-hostile. Instead we do an iterative
@@ -1000,11 +999,11 b' class filectx(basefilectx):'
             # considered when solving linkrev issue are on the table.
             return changectx(self._repo.unfiltered(), self._changeid)
 
-    def filectx(self, fileid):
+    def filectx(self, fileid, changeid=None):
         '''opens an arbitrary revision of the file without
         opening a new filelog'''
         return filectx(self._repo, self._path, fileid=fileid,
-                       filelog=self._filelog)
+                       filelog=self._filelog, changeid=changeid)
 
     def data(self):
         try: