(bug 49434) Support diff=0 in Wikibase

This adds support for requests with diff=0.

Also, moves the revision calculating code and getContent()
out of ViewEntityAction and adds/improves tests.

Ultimately, much of this code should be in core,
with stuff factored out of Article and DifferenceEngine
and merged together with this code, in a new core class.
But best not to block fixing this bug on that for now.

Change-Id: I6d160aa13f12feb98f2e95128e7e7a07ee6831d6
Merge "(bug 49434) Support diff=0 in Wikibase"