Api call changed from .get to .post for getDiff() at Special:Preparation

The api.get method causes problems when the request URL becomes too large
due to bigger source text. This has been fixed by changing the method to .post
as post has no client side restrictions on the data to be sent in the request.

Bug: 69270
Change-Id: Id08208f3494a5c09fc49449b06593e15b4dd5866
Merge "Api call changed from .get to .post for getDiff() at Special:Preparation"