tests/test-issue433

hgweb: don't raise an exception when displying empty repos

The nullid node claims it's in the default branch, but the branch dict
is empty.  This fixes the main symptom from issue696, but we may want
to set branchtags()['default'] = nullid somewhere for empty repos.

#!/bin/sh
# http://www.selenic.com/mercurial/bts/issue433

hg init a
cd a
echo a > a
hg commit -Ama

hg parents -r 0 doesnotexist
true