It's the pre-Gingerbread master branch. See
http://groups.google.com/group/android-building/browse_thread/thread/ea5de03bff285ba8:
"temporarily, a branch called froyo-plus-aosp contains the state of
master as it was before I started the push. If you were working in
master before this morning and find that the gingerbread changes are
causing you trouble, this'll let you temporarily get back to a
pre-gingerbread state."
Jim