extensions: properly mark progress as part of core

This should have been done as part of or as an immediate follow-up to
f5c906878a47, but presumably this feature of extensions.py was
forgotten at that time.

mercurial/extensions.py

@@ -24,8 +24,8 b' from . import ('
 _extensions = {}
 _aftercallbacks = {}
 _order = []
-#  former extensions now in core - we ignore these if found in hgrc
-_builtin = set(['hbisect', 'bookmarks', 'parentrevspec', 'interhg', 'inotify'])
+_builtin = set(['hbisect', 'bookmarks', 'parentrevspec', 'progress', 'interhg',
+                'inotify'])
 
 def extensions(ui=None):
     if ui: