Fix posting submitted changelists for perforce.

The code to post submitted changelists would end up posting a diff  for both
the intended change and the one preceeding it. This is because we passed in
our (base, tip] revisions for 'p4 filelog', which itself behaves as [base,
tip]. To work around this, I've added code to increment the base revision
before calling filelog.

Testing done: Ran unit tests.

Fixes bug 3347.

Reviewed at https://reviews.reviewboard.org/r/5837/