Remove hack: get ref from gerrit

The current implementation of repopick.py has an ugly hack that attempts the merging of patch sets until it can't find a higher versioned one.

This patch modifies the script to determine the latest patch set using Gerrit's REST API: http://review.cyanogenmod.org/Documentation/rest-api-changes.html#get-review

IMO, it's better than the current trial and error method :)