(bug 43820) Content also needed for prop=revisions&rvdiffto=

It was somehow overlooked that prop=revisions&rvdiffto= also requires
that the content for the current revision be fetched.

Change-Id: Ie1d718458da8cdb6ba63bd7ea512db3b299fc1e2
Merge "(bug 43820) Content also needed for prop=revisions&rvdiffto=" into wmf/1.21wmf7