gitweb: ref markers link to named shortlogs

gitweb: ref markers link to named shortlogs

This patch turns ref markers for tags and heads into links to
appropriate views for the ref name, depending on current context.

For annotated tags, we link to the tag view, unless that's the current
view, in which case we switch to shortlog. For other refs, we prefer the
current view if it's history or (short)log, and default to shortlog
otherwise.

Appropriate changes are made in the CSS to prevent ref markers from
being annoyingly blue and underlined, unless hovered. A visual
indication of the target view difference is also implemented by making
annotated tags show up in italic.

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