Option to create a `squash!` commit

[jcorbin]

Sometimes you want to plan to edit the squashed commit message a bit, rather than just speed past it in the future rebase.

To that end, creating a squash! ... commit is super useful, otherwise you'll have to remember to edit the autosquash-prepared rebase plan and change a fixup to a squash.

Fwiw I have a small git-fit wrapper I've been using for years, which can also be invoked as git-sit to quickly bang out such commits; it was only on reading your README that I even learned that git commit --fixup and git commit --squash are a thing.

Supposing in a similar vein, amend! mode could/should also be supported, altho I'm also learning about that mode right now as I read up on git commit --fixup=amend:...

[tummychow]

seems reasonable. choosing which commit type to use on a per-commit basis is probably too difficult, but setting it for an entire git-absorb invocation (as a flag or git config) is a pr i'd accept