magit-commit-for-autosquash: new command

This command makes it possible to create fixup! commits for the autosquash feature of git-rebase. I've used it practically every day for quite some time now and hope it will make amending non-HEAD commits more convenient for others as well. It's of course beneficial to "pool" fixup commits but even if you make just one and apply it immediately, ' E C-x # is a rather quick way to do it (I have ideas to make that even easier but I think this is a reasonable first step).

I'll happily give my blessing for an improvement that also allows squash! commits to be created (with, for example, a prefix argument or so that ' ' creates a fixup! commit and ' s creates a squash! commit), but I've never needed those so whoever does gets to add that feature.