extensions: add shelve to _builtin

This is a follow-up patch to 3de4f17f4824. This adds `shelve` to
`extensions._builtin` so that the shelve extension is silently
ignored.

Differential Revision: https://phab.mercurial-scm.org/D6589

mercurial/extensions.py

@@ -43,7 +43,8 b' from .utils import ('
     'progress',
     'interhg',
     'inotify',
-    'hgcia'
+    'hgcia',
+    'shelve',
 }
 
 def extensions(ui=None):