changing-files: retrieve changelogrevision.files from the sidedata block

The `files` field is know to have issue, using a list with fixed, and fixable,
computation can only help. For example, using a fixes `files` field would be
enough to fix issue6219 once this feature get usable in production.

We focus on having thing working for now, we will deal with performance later.
Right now we have a ironic situation were we parse sorted value from disk to
turn them into a set and then having to sort it again.

Differential Revision: https://phab.mercurial-scm.org/D9092

mercurial/changelog.py

@@ -319,6 +319,8 b' class changelogrevision(object):'
 
     @property
     def files(self):
+        if self._cpsd:
+            return sorted(self.changes.touched)
         off = self._offsets
         if off[2] == off[3]:
             return []