We still need it for empty repo for web UI!

We shouldn't pass a non-existing branch to source_branch.
Checkout test for:

* spec/features/tags/master_views_tags_spec.rb:24
* spec/features/projects/files/project_owner_sees_link_to_create_license_file_in_empty_project_spec.rb:13

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 @ref && @repository.find_branch(@ref)
     commit_params = @commit_params.merge(
       source_project: @project,
-      source_branch: @ref,
+      source_branch: source_branch,
       target_branch: @target_branch
     )