Restore diff_sha_refs method

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

app/models/diff_note.rb

@@ -118,7 +118,11 @@ class DiffNote < Note
   end
 
   def noteable_diff_refs
-    noteable.diff_refs
+    if noteable.respond_to?(:diff_sha_refs)
+      noteable.diff_sha_refs
+    else
+      noteable.diff_refs
+    end
   end
 
   def set_original_position

app/models/merge_request.rb

@@ -265,28 +265,29 @@ class MergeRequest < ActiveRecord::Base
     @source_branch_sha || source_branch_head.try(:sha)
   end
 
-  def branch_merge_base_sha
-    branch_merge_base_commit.try(:sha)
+  def diff_refs
+    return unless diff_start_commit || diff_base_commit
+
+    Gitlab::Diff::DiffRefs.new(
+      base_sha:  diff_base_sha,
+      start_sha: diff_start_sha,
+      head_sha:  diff_head_sha
+    )
   end
 
-  def diff_refs
+  # Return diff_refs instance trying to not touch the git repository
+  def diff_sha_refs
     if merge_request_diff && merge_request_diff.diff_refs_by_sha?
       merge_request_diff.diff_refs
     else
-      start_sha = target_branch_sha
-      head_sha = source_branch_sha
-      base_sha = branch_merge_base_sha
-
-      if start_sha || base_sha
-        Gitlab::Diff::DiffRefs.new(
-          base_sha:  base_sha,
-          start_sha: start_sha,
-          head_sha:  head_sha
-        )
-      end
+      diff_refs
     end
   end
 
+  def branch_merge_base_sha
+    branch_merge_base_commit.try(:sha)
+  end
+
   def validate_branches
     if target_project == source_project && target_branch == source_branch
       errors.add :branch_conflict, "You can not use same project/branch for source and target"
@@ -748,7 +749,7 @@ class MergeRequest < ActiveRecord::Base
   end
 
   def has_complete_diff_refs?
-    diff_refs && diff_refs.complete?
+    diff_sha_refs && diff_sha_refs.complete?
   end
 
   def update_diff_notes_positions(old_diff_refs:, new_diff_refs:)

spec/models/merge_request_spec.rb

@@ -782,7 +782,7 @@ describe MergeRequest, models: true do
     end
   end
 
-  describe "#diff_refs" do
+  describe "#diff_sha_refs" do
     context "with diffs" do
       subject { create(:merge_request, :with_diffs) }
 
@@ -791,7 +791,7 @@ describe MergeRequest, models: true do
 
         expect_any_instance_of(Repository).not_to receive(:commit)
 
-        subject.diff_refs
+        subject.diff_sha_refs
       end
 
       it "returns expected diff_refs" do
@@ -801,7 +801,7 @@ describe MergeRequest, models: true do
           head_sha:  subject.merge_request_diff.head_commit_sha
         )
 
-        expect(subject.diff_refs).to eq(expected_diff_refs)
+        expect(subject.diff_sha_refs).to eq(expected_diff_refs)
       end
     end
   end