(bug 38904) prop=revisions&rvstart=... should not blow up when continuing

(bug 38904) prop=revisions&rvstart=... should not blow up when continuing

A query using prop=revisions in enum mode will return a query-continue
using rvstartid. If the original query included rvstart, the client will
therefore send both the rvstart from the original query and the
rvstartid from the query-continue. And the API will then return an error
that rvstart and rvstartid may not both be used in the same query.

Since there is no way for the API to instruct the client to ''not'' send
the rvstart when continuing, we'll just use rvcontinue instead. Then the
module can ignore both rvstart and rvstartid to get the intended
continuation.

Change-Id: I145215996b8a7818196cdfb583a1fdacae973fee