Merge branch 'zj-compare-source-branch' into 'master'

Migrate compare_source_branch to use only Gitaly

Closes gitaly#771

See merge request gitlab-org/gitlab-ce!19700

lib/gitlab/git/repository.rb

@@ -1181,18 +1181,18 @@ module Gitlab
       end
 
       def compare_source_branch(target_branch_name, source_repository, source_branch_name, straight:)
-        Gitlab::GitalyClient::StorageSettings.allow_disk_access do
-          with_repo_branch_commit(source_repository, source_branch_name) do |commit|
-            break unless commit
+        tmp_ref = "refs/tmp/#{SecureRandom.hex}"
+
+        return unless fetch_source_branch!(source_repository, source_branch_name, tmp_ref)
 
         Gitlab::Git::Compare.new(
           self,
           target_branch_name,
-              commit.sha,
+          tmp_ref,
           straight: straight
         )
-          end
-        end
+      ensure
+        delete_refs(tmp_ref)
       end
 
       def write_ref(ref_path, ref, old_ref: nil, shell: true)