To speed up the build I want to use a refspec to only clone the needed branch like this:
refs/heads/$GIT_LOCAL_BRANCH:refs/remotes/$GIT_BRANCH

This seemed to work great at first, but then I found out this refspec is also used during polling.
This means that not all branches are being built anymore, even though our branch specifier is '**', because they are simply not fetched anymore.

This is why I would like to have an option to specify a different refspec for the build than for the polling. So I can have these refspecs:
polling: refs/heads/*:refs/remotes/*
building: refs/heads/$GIT_LOCAL_BRANCH:refs/remotes/$GIT_BRANCH