Fix parsing of perforce CLNs with multiple sections.

As part of the updates to make code compatible with Python 3, I removed the map
call that was used to do pairwise iteration of the sections in VMware-formatted
changelists, and replaced it with a zip. Unfortunately, map and zip have
different behavior, and this would end up chopping off the last section.

This change replaces it with a much more basic index iteration.

Testing done:
Verified that the iteration method cycled over pairs in the expected fasion.
Sent a patch to VMware for them to verify in production.