Fix Error 500 in creating merge requests with > 1000 diffs

Users trying to create a merge request with > 1000 diffs would get a similar error:

Completed 500 Internal Server Error in 4981ms (ActiveRecord: 34.7ms)
NoMethodError (undefined method `[]' for #<Gitlab::Git::Diff:0x0000000d734648>):
app/models/merge_request_diff.rb:131:in `reload_diffs'
app/models/merge_request_diff.rb:43:in `reload_content'
app/services/merge_requests/create_service.rb:17:in `execute'
app/controllers/projects/merge_requests_controller.rb:110:in `create'

Closes #2692 (closed)

Reassigned to @dzaporozhets

Don't have time now to write a spec, but someone else is welcome to do it.

mentioned in issue #2692 (closed)

mentioned in issue #2572 (closed)

Milestone changed to 8.1

Status changed to merged

Thank you!

mentioned in issue #2572 (closed)

Picked into 8-0-stable.