This avoids hard-coding the '(none)' string in the JavaScript for each
style and avoids the {nonexisting|nonempty} hack which only works
since the template system don't complain about undefined variables.
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.
