Speedup DiffNote#active? on discussions, preloading noteables and avoid touching git repository to return diff_refs when possible

- Preloading noteable we share the same noteable instance when more than one 
discussion refers to the same noteable.
- Any other call to that object that is cached in that object will be for any 
discussion.
- In those cases where merge_request_diff has all the sha stored to build a diff_refs get that 
diff_refs using directly those sha instead accessing to the git repository to first get the 
commits and later the sha.

1 files changed, 6 insertions, 0 deletions

diff --git a/app/models/discussion.rb b/app/models/discussion.rb
index 74facfd1c9c..e2218a5f02b 100644
--- a/app/models/discussion.rb
+++ b/app/models/discussion.rb
@@ -49,6 +49,12 @@ class Discussion
     self.noteable == target && !diff_discussion?
   end
 
+  def active?
+    return @active if defined?(@active)
+
+    @active = first_note.active?
+  end
+
   def expanded?
     !diff_discussion? || active?
   end