undo-files: also remove the undo.backupfiles

The undo.backupfiles is dealt is directly managed by the transaction instead of
going through the `localrepo.undofiles`. We start doing minimal management for
it before using `cleanup_undo_files` on more situation. Proper handling of it
is an intermediate goal of this series.

mercurial/repair.py

@@ -119,7 +119,10 b' def cleanup_undo_files(repo):'
     This is useful to prevent rollback running in situation were it does not
     make sense. For example after a strip.
     """
-    for undovfs, undofile in repo.undofiles():
+    # XXX need to remove the backups themselve too
+    undo_files = [(repo.svfs, b'undo.backupfiles')]
+    undo_files.extend(repo.undofiles())
+    for undovfs, undofile in undo_files:
         try:
             undovfs.unlink(undofile)
         except OSError as e: