gitweb: Add an option for adding more branch refs

Allow extra-branch-refs feature to tell gitweb to show refs from
additional hierarchies in addition to branches in the list-of-branches
view.

Signed-off-by: Krzesimir Nowak <krzesimir@endocode.com>
Reviewed-by: Junio C Hamano <gitster@pobox.com>
Reviewed-by: Jakub Narębski <jnareb@gmail.com>
Reviewed-by: Eric Sunshine <sunshine@sunshineco.com>