This will be used by the frontend to show the banner of the overflow

MR: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/64195

Changelog: added

Add spec and factory for multiple diffs

If merge_request_diff for some reason isn't contained in
@merge_request_diffs, we will see a TypeError

Changelog: fixed

The plan for RSpec 4.0 is to disable monkey patching, reference:
https://rspec.info/blog/2013/07/the-plan-for-rspec-3/#zero-monkey-patching-mode
https://relishapp.com/rspec/rspec-core/docs/configuration/zero-monkey-patching-mode

This commit stops using RSpec monkey patching in several specs.

When group/project/merge_requests/iid/diffs?diff_head=true
is visited, then the diff which compared against HEAD
will be shown

https://gitlab.com/gitlab-org/gitlab/issues/27703