Fix expand all diffs button in compare view
What does this MR do?
Fix the expand all diffs button in compare view.
Are there points in the code the reviewer needs to double check?
Don't think so.
Why was this MR needed?
We can't reuse the existing value of the format parameter, because on
the merge request page that's 'json', so the link would go to a JSON
file. We can't set it to HTML, because that adds '.html' to the URL, which
breaks the compare view (and looks bad). Setting it to
nil uses the
default format, which in all of these cases is HTML anyway.
What are the relevant issue numbers?
Closes #20241 (closed).