filectx.parents: also fetch the filelog of rename source too

we are going to need this filelog for the linkrev adjustment, so we better
normalise the list and have the filelog in all case.

This is done in a previous changeset to help readability.

mercurial/context.py

@@ -749,7 +749,7 b' class basefilectx(object):'
             # As null id have alway been filtered out in the previous list
             # comprehension, inserting to 0 will always result in "replacing
             # first nullid parent with rename information.
-            pl.insert(0, (r[0], r[1], None))
+            pl.insert(0, (r[0], r[1], self._repo.file(r[0])))
 
         return [filectx(self._repo, p, fileid=n, filelog=l) for p, n, l in pl]