Don't create a temp reference for branch comparisons within project

A temp reference is only needed to fetch a branch from another project,
as in the case for forked repositories. For branch comparisons within
the same project, we can just use the existing branch names to do the
comparison.

Relates to https://gitlab.com/gitlab-org/gitlab-ce/issues/38689#note_126107862

changelogs/unreleased/sh-avoid-fetching-temp-refs-within-project.yml

+---
+title: Don't create a temp reference for branch comparisons within project
+merge_request:
+author:
+type: fixed

lib/gitlab/git/repository.rb

@@ -732,6 +732,12 @@ module Gitlab
       end
 
       def compare_source_branch(target_branch_name, source_repository, source_branch_name, straight:)
+        if source_repository == self
+          return unless commit(source_branch_name).present?
+
+          return Gitlab::Git::Compare.new(self, target_branch_name, source_branch_name, straight: straight)
+        end
+
         tmp_ref = "refs/tmp/#{SecureRandom.hex}"
 
         return unless fetch_source_branch!(source_repository, source_branch_name, tmp_ref)
@@ -743,7 +749,7 @@ module Gitlab
           straight: straight
         )
       ensure
-        delete_refs(tmp_ref)
+        delete_refs(tmp_ref) if tmp_ref
       end
 
       def write_ref(ref_path, ref, old_ref: nil)

spec/lib/gitlab/git/repository_spec.rb

@@ -1966,6 +1966,42 @@ describe Gitlab::Git::Repository, :seed_helper do
     end
   end
 
+  describe '#compare_source_branch' do
+    context 'within same repository' do
+      it 'does not create a temp ref' do
+        expect(repository).not_to receive(:fetch_source_branch!)
+        expect(repository).not_to receive(:delete_refs)
+
+        compare = repository.compare_source_branch('master', repository, 'feature', straight: false)
+        expect(compare).to be_a(Gitlab::Git::Compare)
+        expect(compare.commits.count).to be > 0
+      end
+
+      it 'returns nil when source ref does not exist' do
+        compare = repository.compare_source_branch('master', repository, 'non-existent-branch', straight: false)
+
+        expect(compare).to be_nil
+      end
+    end
+
+    context 'with different repositories' do
+      it 'creates a temp ref' do
+        expect(repository).to receive(:fetch_source_branch!).and_call_original
+        expect(repository).to receive(:delete_refs).and_call_original
+
+        compare = repository.compare_source_branch('master', mutable_repository, 'feature', straight: false)
+        expect(compare).to be_a(Gitlab::Git::Compare)
+        expect(compare.commits.count).to be > 0
+      end
+
+      it 'returns nil when source ref does not exist' do
+        compare = repository.compare_source_branch('master', mutable_repository, 'non-existent-branch', straight: false)
+
+        expect(compare).to be_nil
+      end
+    end
+  end
+
   describe '#checksum' do
     it 'calculates the checksum for non-empty repo' do
       expect(repository.checksum).to eq '51d0a9662681f93e1fee547a6b7ba2bcaf716059'