Try if branch_exists? would work, feedback:

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/7237#note_18424135

app/controllers/concerns/creates_commit.rb

@@ -4,7 +4,7 @@ module CreatesCommit
   def create_commit(service, success_path:, failure_path:, failure_view: nil, success_notice: nil)
     set_commit_variables
 
-    source_branch = @ref if @ref && @repository.find_branch(@ref)
+    source_branch = @ref if @ref && @repository.branch_exists?(@ref)
     commit_params = @commit_params.merge(
       source_project: @project,
       source_branch: source_branch,