Completing support for Perforce default changeset

Review Request #1418 — Created Feb. 17, 2010 and submitted — Latest diff uploaded

rbtools, reviewboard
Reading the release notes, I was happily surprised to see that P4 default changeset is supported by post-review.

However, a small fix is required to make it work for more recent version of P4.  The test on line 1639 is incomplete.
Tested on my local staging server, using 0.2RC1.
Posted this review with the updated version, though it should not be relevant, as this is specific to Perforce.