Respect pre-commit hooks

[mgehre-amd]

This is a great tool that I had waiting to have for last couple of years. Thanks!

What I currently see is the following:

1. I make a change with a formatting error
2. I do git add -p to stage it
3. I do git fixup to fix it up
4. git fixup tries to create a commit, but that fails due to my pre-commit hooks
5. git fixup still calls interactive rebase, which now shows an editor with the rebase instructions; but there is no fixup commit here
6. I save the interactive rebase editor, the no-op rebase happens, and I'm back. I now go and commit the formatting corrections.

It would be great if git-fixup would stop at 4. if the pre-commit hooks fail, and doesn't run the interactive rebase anyways.