commands: rewrite legacy ternary operator hack using modern syntax

Differential Revision: https://phab.mercurial-scm.org/D1885

mercurial/commands.py

@@ -1219,7 +1219,7 b' def bundle(ui, repo, fname, dest=None, *'
             raise error.Abort(_("--base is incompatible with specifying "
                                "a destination"))
         common = [repo.lookup(rev) for rev in base]
-        heads = revs and map(repo.lookup, revs) or None
+        heads = map(repo.lookup, revs) if revs else None
         outgoing = discovery.outgoing(repo, common, heads)
     else:
         dest = ui.expandpath(dest or 'default-push', dest or 'default')