New SHA widget breaks on "New Merge Request"

Summary

It seems like https://gitlab.com/gitlab-org/gitlab-ce/issues/42448 introduced a regression on the "New Merge Request" page. :

(Thanks @winh for finding!)

/cc @dimitrieh

Steps to reproduce

1. Go to https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/new#
2. pick a branch

Possible fixes

Removing the rule

.merge-request-branches .commit-detail {
    flex-direction: column;
}

results in this behavior