hgweb: fix empty navigation detection
For some obscure reason, changelog.node(0) returns nullid if changelog is empty.
this break empty navigation detection. We fix this code by using the length of
the changelog.
Using the length have some issue with revision filtering but this is a small
step in the right direction. Proper fix comes in later changeset.
$ "$TESTDIR/hghave" pyflakes || exit 80
$ cd "`dirname "$TESTDIR"`"
$ pyflakes mercurial hgext 2>&1 | "$TESTDIR/filterpyflakes.py"
hgext/inotify/linux/__init__.py:*: 'from _inotify import *' used; unable to detect undefined names (glob)