Squash and merge behaviour when dealing with a single commit

Currently our docs call out how the metadata is set for the squashed commit here: https://docs.gitlab.com/ee/user/project/merge_requests/squash_and_merge.html#commit-metadata-for-squashed-commits

Specifically when squashing commits the commit message becomes the title of the merge request.

We make it clear, either in the merge widget, or in the Docs, the fact that if there is only one commit, there is nothing to squash and we don't re-write the single commit to match this same behaviour.

This was run into recently in a support issue: https://gitlab.zendesk.com/agent/tickets/82781