Inconsistent behaviour for nextPageStart attribute on commits REST resource when specifying a since parameter

This is due to the fact that we use our streaming API (and paginate it in Java) when there is a since parameter but use the Paged API when there isn't. We need to modify com.atlassian.stash.rest.pull.JsonChangesetCallback so that it passes back a nextPageStart

Raised from https://answers.atlassian.com/questions/151325/why-is-islastpage-false-but-no-nextpagestart