Basic atom feed test
Include diff in Atom feed

As cgit was not prepared to produce multiple diffs per run,
so this also reset some counters during the diff. Additionally,
the cgit_self_link had to be extended, as it is called during
the diffing.
Improved readability.
Skip forbidden characters.
div->pre for better default formatting
colorize diff deletes and inserts
added configuration option for diff in atom feed
add xml-header to atom-feed
display updated, use urn/url for atom:id
only one content tag allowed, so remove the text one
add <link rel='self'> to atom feed

FEED Validator recommends it.
fix html error in diff output for binary files