improve auto merge commit message

In our pipeline we use auto-merge after CI/CD success. We squash and delete branches.

Sometimes the squash commit message picks randomly a previous commit in the branch. Not the last everytime. It's quite annoying. That's why most of the time we change the commit message manually to use the MR title for example.

We suggest 4 improvements toward the auto-merge and especially commit message:

• Add a select picker to choose previous commit to use as squash commit message - already there
• Add a checkbox to use MR title as squash commit message
• Once we have enabled auto merge it's not written anywhere the branch will be squashed. A message like The commits will be squashed could be placed after The changes will be merged into BRANCH
• Once we have define a commit message and enabled auto merge, we should have a way to view what will be the commit message. Without this preview, to ensure proper commit message was entered, we must cancel auto-merge then rewrite message then re-enable auto merge.

For this last suggestion, it could be worded as follow: The changes will be merged into BRANCH with following commit message: THE COMMIT MESSAGE HERE. We could also use a tooltip.