Don't pass source_branch if it doesn't exist

app/controllers/concerns/creates_commit.rb

@@ -4,9 +4,10 @@ module CreatesCommit
   def create_commit(service, success_path:, failure_path:, failure_view: nil, success_notice: nil)
     set_commit_variables
 
+    source_branch = @ref if @repository.find_branch(@ref)
     commit_params = @commit_params.merge(
       source_project: @project,
-      source_branch: @ref,
+      source_branch: source_branch,
       target_branch: @target_branch
     )