Fix 404 bug
What does this MR do and why?
this MR for fixing similar issue as #23841.
if it has no diff between the version with merge branch then we visit
diffs_metadata.json?diff_head=true&diff_id=3&start_sha=860fc39564f1102e42a7b3b5deacba251bb259ac&view=inline&w=0
it return 404
Screenshots or screen recordings
Steps to reproduce
1.create one merge request
2.change the commit file same with branch will merge
3.click compare with previous version
Describe something about code
1.the code file is app/controllers/projects/merge_requests/diffs_controller.rb
2.remove viewable which is query condition from the function find_merge_request_diff_compare
in the line @merge_request.merge_request_diffs.viewable.find_by(id: params[:diff_id]) to fix 404 bug
3.only one function call find_merge_request_diff_compare
4.origin author may think that the mr's commit file must not be same with the branch that will merge, but it is not
after remove viewable pipeline is passed。 @daveliu