Merge When Build Succeeds doesn't work when FF merge is impossible

Customer reported that MWBS does not work when FF merge is not possible. See https://gitlab.com/gitlab-org/gitlab-ce/issues/21138#note_18023472.

Steps to reproduce:

• Create a MR and start running the pipeline, set the MR to "Auto merge when build succeeds"
• Push another commit directly to master, and having "only allow FF merges"
• Build finishes successfully, but now can't merge because it isn't a FF merge.

The MR is stuck like the image above - no "rebase" option, only the "cancel automatic merge".

Workaround

User has to:

• Cancel Merge When Build Succeeds
• Refresh the page
• Click rebase button
• Accept the merge request

Solution

Add a TODO when MWBS was unsuccessful, cancel MWBS automatically.