gitweb: allow extra breadcrumbs to prefix the trail

There are often parent pages logically above the gitweb projects
list, e.g. home pages of the organization and department that host
the gitweb server. This change allows you to include links to those
pages in gitweb's breadcrumb trail.

Signed-off-by: Tony Finch <redacted>
Reviewed-by: Jonathan Nieder <redacted>
Acked-by: Jakub Narebski <redacted>
Signed-off-by: Junio C Hamano <redacted>