Support merge train MR ref being created with rebase

Proposal

From spike: #390937 (comment 1321350070)

Step 2: Our current ref is created using a merge commit but this won't work for fast forward merging. In order to support FF merge with merge trains we need to use a ref that is created via rebasing. The merge train merge request ref should be made by rebasing master and the previous branches on to a ref made from the target branch master + target branch + previous branches. via rebase.