Fix auto-MR-close text from branch name
## What does this MR do?
Fix the MR-auto-close feature if a branch name begins with an issue IID.
## Are there points in the code the reviewer needs to double check?
It's one line, so check that line thoroughly 😃
## Why was this MR needed?
When I create a new MR from a branch starting with an issue IID, the description doesn't get suffixed with the 'Closes #blahblah' that it should do.
## What are the relevant issue numbers?
Don't think there are any.
## Screenshots (if relevant)
## Does this MR meet the acceptance criteria?
- [ ] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
See merge request !4836
a3a8798a1b