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, 2 insertions, 2 deletions

diff --git a/spec/models/diff_note_spec.rb b/spec/models/diff_note_spec.rb
index af8e890ca95..1fa96eb1f15 100644
--- a/spec/models/diff_note_spec.rb
+++ b/spec/models/diff_note_spec.rb
@@ -119,7 +119,7 @@ describe DiffNote, models: true do
 
       context "when the merge request's diff refs don't match that of the diff note" do
         before do
-          allow(subject.noteable).to receive(:diff_refs).and_return(commit.diff_refs)
+          allow(subject.noteable).to receive(:diff_sha_refs).and_return(commit.diff_refs)
         end
 
         it "returns false" do
@@ -168,7 +168,7 @@ describe DiffNote, models: true do
 
         context "when the note is outdated" do
           before do
-            allow(merge_request).to receive(:diff_refs).and_return(commit.diff_refs)
+            allow(merge_request).to receive(:diff_sha_refs).and_return(commit.diff_refs)
           end
 
           it "uses the DiffPositionUpdateService" do