Bug: Show an error when using the /merge quick action with an out of date head sha
We do a check in the merge service (https://gitlab.com/gitlab-org/gitlab/-/blob/d14b5c3f85e8c88565c5ad8d6e1bf182a29f7105/app/services/merge_requests/merge_service.rb#L155) to see if the sources matches while locking. But we should also do another check earlier in the process without locking to eliminate the majority of cases where this may occur and return a more appropriate error.
Edited by Marc Shaw