Autosquash makes it quicker and easier to squash or fixup commits during an
interactive rebase. It can be enabled for each rebase using `git rebase -i
--autosquash`, but it's easier to turn it on by default.

Say I have this history:

    $ git log --oneline
    aaa1111 A first commit
    bbb2222 A second commit
    ccc3333 A third commit

I make another change that I already know should be squashed into "A
second commit". I can do this:

    $ git add .
    $ git commit --squash bbb2222
    [my-branch ddd4444] squash! A second commit

Then when I rebase:

    $ git rebase -i origin/my-branch

The interactive rebase list will be set up ready to squash:

    pick aaa1111 A first commit
    pick bbb2222 A second commit
    squash ddd4444 squash! A second commit
    pick ccc3333 A third commit

Since it's unlikely that anyone will be writing a commit message that begins
`squash!` or `fixup!` when they don't want this behaviour, and the user
still has a chance to review what's going to happen with the rebase, it's
safe to have it always turned on.