At least subversion allows empty commit logs which, after conversion,
show up as empty fields in hgweb and don't allow selecting the
changeset. Using "nonempty" as filter, "(none)" is used to get
clickable links for empty commit messages.
This removes the ability for templates to add custom HTTP headers, which can
easily be re-added if someone needs it. Thanks to asak for repeatedly reviewing
this patch and helping to iron out the quirks.
