build - Add git gc --aggressive to src and pkgsrc git repo creation

build - Add git gc --aggressive to src and pkgsrc git repo creation

* Issue a git gc --aggressive after pulling the git repo for src
  and/or pkgsrc.  This will garbage collect and recompress the
  repo, significantly reducing the disk space required.

* Primarily intended to reduce the size of the usb and dvd gui nrelease
  images.

* The gc pass takes a few minutes to run but seems worth it.