Improve squash on merge UI as it can be unexpected

Summary

While trying to merge a few commits (that I had just nicely split up), I hit the big blue "Merge when pipeline succeeds" button. However, the "Squash commits" checkbutton must have been set, something that I simply did not see. Which really isn't surprising as I never ever use that feature (I didn't even realise it exists).

I feel the UI should be improved to make this more obvious. e.g.:

• Make it the mergers choice what to do, not the submitters
• Let the button explicitly state the action "Squash and merge […]"
• Add a setting to disallow users to even set it
• Only open the commit message expander, then have a second button below (i.e. making it a two step action and also getting a confirmation for the commit message).

Not sure … I am obviously a unhappy as it caught me completely by surprise. Below is my original report.

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A merge with three commits was squashed during while merging. The commit message of the middle commit was kept, but the repository is set to do fast-forward merges only.

Steps to reproduce

I did the following, have not confirmed if it is reproducible:

• Checked out fork'ed repository (not mine)
• Rewrote the history to split a commit (and change a comment)
• Force pushed into the user repository
• Rebased from the MR
• Merged

This is the MR in question, you'll see the three commits:

• https://gitlab.freedesktop.org/libfprint/libfprint/-/merge_requests/144
• 5f71cc83 - vfs0050: Attach SSM to transfer to fix an assertion
• f0c34e1a - vfs0050: Stop capture after a timeout happens
• 679a49d8 - vfs0050: Fix byte offset calculation into struct

The pipeline ran through for 679a49d8, and it was then merged. But the merged commit is 1b74813a which has the commit message from f0c34e1a and actually is a squashed version of my commits (not the submitters, then one of the comments would be different). So the code is correct, but the commit history got mangled.

The project is set to only allow fast-forward merges.