Stop returning mr head diffs as regular diffs

Issue: Merge head commit set as `last_commit` in merge... (#578761)

What does this MR do and why?

References

This MR attempts to fix a bug that is related to a RFH.

gitlab-com/request-for-help#3510+

I believe there is some caching of merge_head diffs, due to the relation specification with the merge_request model. I do not believe the scope on the has_one is sufficent to distinguish between the two different kinds of diffs, to demonstrate how this bug could occur run via rails console:

mr = MergeRequest.find(mr_id) # find an MR with a merge_head diff
mr.merge_request_diff.diff_type # this should give "regular"
mr.merge_head_diff.head_commit # this call causes the mr.merge_request_diff to cache the `merge_head_diff` as the `merge_request_diff`
mr.merge_request_diff.diff_type # this would produce the merge_head_diff

Setting inverse_of to false is one way of preventing Rails from caching the merge_head diff, as the merge_request_diff.

Screenshots or screen recordings

Before	After

How to set up and validate locally

mr = MergeRequest.find(mr_id) # find an MR with a merge_head diff
mr.merge_request_diff.diff_type # this should give "regular"
mr.merge_head_diff.head_commit # load a merge_head_diff
mr.merge_request_diff # this should still be the same regular diff, with the same ID

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