Return the right commit range for pull requests

The compare URL for pull requests is just the pull request URL, so we can't extract the commit range from it, but the commit range is stored in the request for pull requests, so we can get it from there.

Fixes travis-ci/travis-ci#1719.
Does this mean the base commit of the most recent push, or the base commit of the entire pull request? I'm trying to understand what would happen if I pushed additional commits to a pull request