gitweb: Allow for href() to be used for links without project param

gitweb: Allow for href() to be used for links without project param

Make it possible to use href() subroutine to generate link with
query string which does not include project ('p') parameter.
href() used to add project=$project to its parameters, if it
was not set (to be more exact if $params{'project'} was false).
Now you can pass "project => undef" if you don't want for href()
to add project parameter to query string in the generated link.

Links to "project_list", "project_index" and "opml" (all related
to list of all projects/all git repositories) doesn't need project
parameter. Moreover "project_list" is default view (action) if
project ('p') parameter is not set, just like "summary" is default
view (action) if project is set; project list served as a kind
of "home" page for gitweb instalation, and links to "project_list"
view were done without specyfying it as an action.

Convert remaining links (except $home_link and anchor links)
to use href(); this required adding 'order => "o"' to @mapping
in href(). This finishes consolidation of URL generation.

Signed-off-by: Jakub Narebski <jnareb@gmail.com>
Signed-off-by: Junio C Hamano <junkio@cox.net>