Fix condensed range in MR push comment.

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/1246

In https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/326#note_984907, you see that the compare page doesn't actually have 39 commits because we're using `first_new_commit^`, which is not always the same as `oldrev`. With this MR, we use `oldrev` instead, which will always work.

See merge request !1721