gitweb: Mark matched 'ctag' / contents tag (?by_tag=foo)

gitweb: Mark matched 'ctag' / contents tag (?by_tag=foo)

It might have been hard to discover that current view is limited to
projects with given content tag (ctag), as it was distinquished only
in gitweb URL.  Mark matched contents tag in the tag cloud using
"match" class, for easier discovery.

This commit introduces a bit of further code duplication in
git_populate_project_tagcloud().

Signed-off-by: Jakub Narebski <jnareb@gmail.com>
Acked-by: Petr Baudis <pasky@suse.cz>
Signed-off-by: Junio C Hamano <gitster@pobox.com>