[PATCH 0 of 2] hgweb: Allow empty changelog messages (resend)

[Rocco Rutte]

In many templates the changelog message is a link to the changeset. In
case there is no changelog message (which subversion allows) there's no
easy way to get the info (except choosing a child changeset and use
parent links).

These patches add a "nonempty" filter and use in templates where needed
to print "(none)" instead to get a clickable link.