gs commit float, sink: Send a commit up or down the stack

The `git commit float` and `git commit sink` commands will allow sending a commit from the current branch up or down the stack.

Things to decide:

1. Which commits can be moved like this?
2. Can you float/sink to any upstack/downstack branch, or only the next one?

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For (1), my first reaction is:
`git commit float` can only float the top-most commit of the branch,
and `git commit sink` can only sink the bottom-most commit of the branch,
with the idea that if a commit in the middle of the branch has to be moved,
you should `gs branch edit` to move it to one of the extremes first.

For (2), I think you should be able to select any of the upstack/downstack branches.

Opinions welcome.