gitweb: allow action specialization in page header

gitweb: allow action specialization in page header

An optional -action_extra parameter is given to git_header_html() to
identify a variant of the action that is being displayed. For example,
this can be used to specify that the remotes view is being used for a
specific remote and not to display all remotes.

When -action_extra is provided, the action name in the header will be
turned into a link to the action without any arguments or parameters, to
provide a quick link to the non-specific variant of the action.

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