Check push rules against all commits that have diverged before merging

Description

• Check push rules against all commits that have diverged in the merge request when attempting to merge it.
• In particular, check only the commits that are in the commits tab of the merge request.
• If any of the commits fails any push rules of the target branch's project, then stop the merge.
• Pending some UI flow to indicate the merge failed.

Original description

Zendesk: https://gitlab.zendesk.com/agent/tickets/50264

A customer has been able to reliably reproduce an issue where a merge request can be accepted despite the commit message not conforming to the commit message push rule. They also reproduced it on GitLab.com in their own test project (and I confirmed this is true myself), but I cannot reproduce the same issue in my own test project.

Here are the steps to reproduce (sometimes):

1. Commit to a branch when the Push Rule is disabled
2. Add the <log>.*<\/log> push rule. This is the exact regex the customer is using - unsure if it's relevant as I couldn't reproduce outside their test project.
3. Turn on approvals for the project. (This may or may not be relevant, also - but including here for posterity)
4. Approve the merge request.
5. Attempt to merge the merge request.

When reproducible, the 5th step would result in a successful merge.

The customer has created a test project where we can see/reproduce the issue at jogux/commitmessagetest!3 (merged).