gitweb: Make showing branches configurable

Running 'make GITWEB_WANTED_REFS="heads wip" gitweb.cgi' will create a
gitweb CGI script showing 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>