gitweb: Add an option for adding more branch refs

Overriding an @additional_branch_refs configuration variable with
value ('wip') will make gitweb to show branches that appear in
refs/heads and refs/wip (refs/heads is hardcoded). Might be useful for
gerrit setups where user branches are not stored under refs/heads/.

Signed-off-by: Krzesimir Nowak <krzesimir@endocode.com>