Allow pushing locked LFS files merged from upstream

What does this MR do and why?

Allow pushing locked LFS files merged from upstream

Fixes Git LFS lock in GitLab prevents pushes of new f... (#23625 - closed)

Closes [Feature flag] Rollout of `merge_commit_diff_mo... (#415579 - closed)

Prior to this fix, if a user locked a file and then another user merged a branch into theirs and then requested to merge into the original branch our LFS lock checks would reject this push.

We've introduced merge_commit_diff_modes to ensure we do not include changes from merge commits when checking for locked files.

This will still fail if the merge includes any amended files or conflicts where we didn't pick theirs.

Changelog: fixed