Add diffs streaming endpoint for MR creation page

This is just like Add diffs streaming endpoint for compare (#451171 - closed) but this one is for the "changes" we show in MR creation page.

The code design should be almost the same and the resource would still be a MergeRequest but it should be getting the diffs from a Compare object.

This will require the following:

• Modifying MergeRequest#diffs_for_streaming to get the diffs from #compare. This attribute gets set in MergeRequests::BuildService which is a Compare object.
• Implementing a diffs stream endpoint specific for MR creation page.