bundle: consistently put revnums in "base" collection

The "base" collection contains revnums, except that it can also
contain the "null" symbol. That's a little weird. Let's be consistent.

Differential Revision: https://phab.mercurial-scm.org/D4779

mercurial/commands.py

@@ -1224,7 +1224,7 b' def bundle(ui, repo, fname, dest=None, *'
                                 "a destination"))
         if opts.get('base'):
             ui.warn(_("ignoring --base because --all was specified\n"))
-        base = ['null']
+        base = [nullrev]
     else:
         base = scmutil.revrange(repo, opts.get('base'))
     if cgversion not in changegroup.supportedoutgoingversions(repo):