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
"$TESTDIR/hghave" git || exit 80
echo "[extensions]" >> $HGRCPATH
echo "convert=" >> $HGRCPATH
mkdir git-repo
cd git-repo
git init-db >/dev/null 2>/dev/null
echo a > a
git add a
git commit -m t1 >/dev/null 2>/dev/null || echo "git commit error"
echo b >> a
git commit -a -m t2 >/dev/null || echo "git commit error"
cd ..
hg convert git-repo