Drupal appears to re-roll modules from cvs on a nightly basis in such
a way that the file changes every time so distribute a stable copy via
MASTER_SITE_LOCAL. Since the file version doesn't change even when the
contents change (for instance a recent set of commits resulting in no
changes except an edition of a blank line and a new cvs Id), store the
date of the snapsnot in PORTREVISION.
Reported by: kris