gitweb: Make showing branches configurable

Overriding a @branch_refs variable in config file with value ('heads',
'wip') will make gitweb to show branches that appear in refs/heads/
and in refs/wip/. Might be useful for gerrit setups where user
branches are not stored under refs/heads/.

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