Hide squash checkbox when there is a single commit
When there is only one commit, the squash checkbox doesn't do anything. When a commit is added after the widget is loaded, the MR can't be merged anyway - you have to refresh the page. So we can only show the checkbox on page load if there's more than one commit.
Here's a really badly-made gif, because I can't use computers good.
No changelog needed, as this is a new feature. Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/1697.