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.
Basic install:
$ make # see install targets
$ make install # do a system-wide install
$ hg debuginstall # sanity-check setup
$ hg # see help
See http://www.selenic.com/mercurial/ for detailed installation
instructions, platform-specific notes, and Mercurial user information.