Maintain commit order in MRs

`Gitlab::Git::Compare` will already have the correct order; sorting in
Ruby can only ruin that. (The correct order being the same as `git log`
- reverse chronological while maintaining the commit graph.)

As an example, imagine a branch where someone has their system clock set
wrong for some of the commits. Not only will those commits be in the
wrong order - which is maybe reasonable - but sorting in Ruby can also
put commits with the same timestamp out of order, as Ruby's sorting
isn't stable.

app/models/merge_request_diff.rb

@@ -98,9 +98,7 @@ class MergeRequestDiff < ActiveRecord::Base
     commits = compare.commits
 
     if commits.present?
-      commits = Commit.decorate(commits, merge_request.source_project).
-        sort_by(&:created_at).
-        reverse
+      commits = Commit.decorate(commits, merge_request.source_project).reverse
     end
 
     commits