Merge branch '20241-expand-all-button-breaks-diff-comparison-view' into 'master'

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.

See merge request !5500
Signed-off-by: Rémy Coutable <remy@rymai.me>

CHANGELOG

@@ -15,6 +15,7 @@ v 8.10.2 (unreleased)
   - Added `*.js.es6 gitlab-language=javascript` to `.gitattributes`. !5486
   - Don't show comment button in gutter of diffs on MR discussion tab. !5493
   - Rescue Rugged::OSError (lock exists) when creating references. !5497
+  - Fix expand all diffs button in compare view. !5500
 
 v 8.10.1
   - Refactor repository storages documentation. !5428

app/views/projects/diffs/_diffs.html.haml

@@ -7,7 +7,7 @@
 .content-block.oneline-block.files-changed
   .inline-parallel-buttons
     - if !expand_all_diffs? && diff_files.any? { |diff_file| diff_file.collapsed? }
-      = link_to 'Expand all', url_for(params.merge(expand_all_diffs: 1, format: 'html')), class: 'btn btn-default'
+      = link_to 'Expand all', url_for(params.merge(expand_all_diffs: 1, format: nil)), class: 'btn btn-default'
     - if show_whitespace_toggle
       - if current_controller?(:commit)
         = commit_diff_whitespace_link(@project, @commit, class: 'hidden-xs')