Do not use MergeRequest#commits in define_pipelines_vars

## What does this MR do?

`MergeRequest#commits` requires merge_request_diff to be present.
This is not the case when creating a new merge request.

## What are the relevant issue numbers?

Fix up for https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/7568

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/24798

See merge request !7664