Fix old MR diffs

What does this MR do?

Fix old MR diffs, when we can't figure out the correct diff refs. This is in two commits. The first doesn't preview them, and the second makes them look like this (note that the added diff can still be highlighted!):

Are there points in the code the reviewer needs to double check?

1. Should new_file? || deleted_file? || content_changed? be moved to a method? What should we call it?
2. I didn't actually do https://gitlab.com/gitlab-org/gitlab-ce/issues/36516#note_38068227, but just redefined highlighted_diff_lines instead. This seems more natural to me, but maybe I'm missing something.

Why was this MR needed?

I broke fallback_diff_refs

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests added for this feature/bug
• Review
  • Has been reviewed by Backend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/36516.

To test this, I did:

diff --git a/lib/gitlab/diff/file.rb b/lib/gitlab/diff/file.rb
index f061753f4e..64cab6b0a8 100644
--- a/lib/gitlab/diff/file.rb
+++ b/lib/gitlab/diff/file.rb
@@ -98,6 +98,7 @@ module Gitlab

       def old_blob
         return @old_blob if defined?(@old_blob)
+        return @old_blob = new_blob if new_blob

         sha = old_content_sha
         return @old_blob = nil unless sha
diff --git a/lib/gitlab/diff/file_collection/merge_request_diff.rb b/lib/gitlab/diff/file_collection/merge_request_diff.rb
index fcda1fe223..21ca90dd69 100644
--- a/lib/gitlab/diff/file_collection/merge_request_diff.rb
+++ b/lib/gitlab/diff/file_collection/merge_request_diff.rb
@@ -30,9 +30,9 @@ module Gitlab
         end

         def highlight_diff_file_from_cache!(diff_file, cache_diff_lines)
-          diff_file.highlighted_diff_lines = cache_diff_lines.map do |line|
-            Gitlab::Diff::Line.init_from_hash(line)
-          end
+          # diff_file.highlighted_diff_lines = cache_diff_lines.map do |line|
+          #   Gitlab::Diff::Line.init_from_hash(line)
+          # end
         end

         #

restored source branch fix-old-mr-diffs

1. I didn't actually do https://gitlab.com/gitlab-org/gitlab-ce/issues/36516#note_38068227, but just redefined highlighted_diff_lines instead. This seems more natural to me, but maybe I'm missing something.

@smcgivern It think we should be able to use Diff#content_changed? regardless of whether there are blobs or not. Right now, the result is clearly wrong.

assigned to @smcgivern

resolved all discussions

@DouweM thanks, those reasons make sense. I've updated this now to just touch #content_changed?

assigned to @DouweM

added 1 commit

• 6d372d9d - Show un-highlighted diffs when blobs are the same

Compare with previous version

assigned to @smcgivern

assigned to @DouweM

added 1 commit

• 3d7bce91 - Show un-highlighted diffs when blobs are the same

Compare with previous version

assigned to @smcgivern

resolved all discussions

added 1 commit

• e8525e10 - Show un-highlighted diffs when blobs are the same

Compare with previous version

@DouweM thanks, updated to that - I agree that's clearer on the cause

assigned to @DouweM