Introduce merge_request.squash git push option

When creating a new MR from a git push, I'd like to be able to specify that the commits should be squashed when the MR is merged. Currently, the commits are not squashed by default and I must edit the resulting MR, or use an extra API call to set this option. We should add a merge_request.squash option to https://docs.gitlab.com/ee/gitlab-basics/add-file.html#push-options-for-merge-requests.