Historically, the -mm tree focused on new developments for the memory management part of the kernel (mm).
Occasionally, the -mm tree was overloaded with new patches, so testing it became difficult.
The presence of the new linux-next git repository has offloaded much of the work that made mm maintenance troublesome, allowing Morton to continue to use quilt to manage his series of "mmotm" (mm of the moment) patches.
Morton includes a subset of the mmotm patches in linux-next, which has a head called "linux-next/akpm".
To get all of the mm patches at any given time, developers still need quilt or ad hoc shell scripts to apply the full set of patches.