Tell people to rebase themselves when ee-specific-lines-check failed to rebase

In https://gitlab.com/gitlab-org/gitlab-ee/issues/6038#note_86862115 we eventually identified that conflicts could happen whenever CE branch is up-to-date with CE master with merge and resolved conflicts for at least once.

We also realized that it's very hard to solve this problem at the moment.

We should just tell people to rebase if they want to see this pass for now.