Add a few comments to explain implementation detail

app/models/repository.rb

@@ -1121,6 +1121,8 @@ class Repository
     fetch_ref(path_to_repo, ref, ref_path)
   end
 
+  # Whenever `source_branch` is passed, if `branch` doesn't exist, it would
+  # be created from `source_branch`.
   def update_branch_with_hooks(current_user, branch, source_branch: nil)
     update_autocrlf_option
 

app/services/files/base_service.rb

@@ -75,6 +75,9 @@ module Files
       validate_target_branch
 
       if @source_project != project
+        # Make sure we have the source_branch in target project,
+        # and use source_branch as target_branch directly to avoid
+        # unnecessary source branch in target project.
         @project.fetch_ref(@source_project, @target_branch, @source_branch)
       end
     end