handle squash/rebase merges without --force

[casperdcl]

If you always squash-merge, the unsafe --force flag is always required.

Instead, ideally gh-clean-branches should do one of:

• GH API query to get the commit hash just before it was merged (check that it's identical to or ahead of the local branch)
• try to merge main into the local branch (and check that the diff is empty)

[davidraviv]

Thanks for the suggestion!
I'll have a look at it. Meanwhile, if you feel like contributing a PR it would be great :)