"Closes <issue>" in MR description doesn't close when MR is squashed and merged

Description

Observed on .com over here #281812 (comment 451309056)

@winniehell created an MR over here !47582 (merged), but after merge, the related issue wasn't closed.

Investigation

I hit "squash" before merge and you see that the "Closes" aren't added to the commit message then 7178e65a.

Since there's no commit messages that contain the "Closes", then it looks like we never process this https://gitlab.com/gitlab-org/gitlab/blob/bf306df38f64fc0ac6e9e815b0e5a7ce70809fff/app%2Fworkers%2Fprocess_commit_worker.rb#L60.