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