Fix bug where wrong commit ID was being used in a merge request diff to show old image

Closes #3769

Closes #14327

CHANGELOG

 Please view this file on the master branch, on stable branches it's out of date.
 v 8.6.0 (unreleased)
+  - Fix bug where wrong commit ID was being used in a merge request diff to show old image (Stan Hu)
   - Add confidential issues
   - Bump gitlab_git to 9.0.3 (Stan Hu)
   - Support Golang subpackage fetching (Stan Hu)

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

@@ -20,4 +20,4 @@
     - next unless blob
     = render 'projects/diffs/file', i: index, project: project,
-      diff_file: diff_file, diff_commit: diff_commit, blob: blob
+      diff_file: diff_file, diff_commit: diff_commit, blob: blob, diff_refs: diff_refs

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

@@ -53,6 +53,6 @@
           = render "projects/diffs/text_file", diff_file: diff_file, index: i
       - elsif blob.image?
         - old_file = project.repository.prev_blob_for_diff(diff_commit, diff_file)
-        = render "projects/diffs/image", diff_file: diff_file, old_file: old_file, file: blob, index: i
+        = render "projects/diffs/image", diff_file: diff_file, old_file: old_file, file: blob, index: i, diff_refs: diff_refs
       - else
         .nothing-here-block No preview for this file type

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

 - diff = diff_file.diff
 - file_raw_path = namespace_project_raw_path(@project.namespace, @project, tree_join(@commit.id, diff.new_path))
-- old_file_raw_path = namespace_project_raw_path(@project.namespace, @project, tree_join(@commit.parent_id, diff.old_path))
+- old_commit_id = diff_refs.first.id
+- old_file_raw_path = namespace_project_raw_path(@project.namespace, @project, tree_join(old_commit_id, diff.old_path))
 - if diff.renamed_file || diff.new_file || diff.deleted_file
   .image
     %span.wrap
@@ -12,7 +13,7 @@
     %div.two-up.view
       %span.wrap
         .frame.deleted
-          %a{href: namespace_project_blob_path(@project.namespace, @project, tree_join(@commit.parent_id, diff.old_path))}
+          %a{href: namespace_project_blob_path(@project.namespace, @project, tree_join(old_commit_id, diff.old_path))}
             %img{src: old_file_raw_path}
         %p.image-info.hide
           %span.meta-filesize= "#{number_to_human_size old_file.size}"