Automatically rebase and retry merge when fast-forward MR is no longer possible

Problem to solve

Situation:

• Project is set to Fast-Forward only
• A merge request is set to "Merge when pipeline succeeds"
• Another merge request is merged causing the previous MR to be "no longer possible" to merge

The user then has to go back to the MR and manually rebase and set "Merge when pipeline succeeds" again.

Proposal

One of two options:

1. Rebase automatically, if possible. Then merge.
2. Rebase automatically, if possible. Then rerun the CI, then merge.

I'm not sure if every user will want this setting, but it could be a project admin setting to enable.

This issue is based on a user comment on a previous issue: #16388 (comment 228229605)