Master-side git source step always uses default branch after clone

[Tobi]

Master-side git source step doesn't respect the branch property of the build request when has to do a full clone.

This means that for all modes/methods, it may build the wrong branch if the source dir is not updateable, and in particular full/clobber will always build the default branch, regardless of the branch property.

This happens because the master-side git source step only does a git clone if source is missing: it does not checkout any branch in particular, so it will happen to build the branch git clone checked out (typically, master).