amend: new extension providing the amend command
Various third parties have implemented the `amend` command, which is in high
demand. This patch adds it as an experimental extension so its interface
could be formalized in core directly.
Since `commit --amend` is basically what `amend` should do. The command is
just a thin wrapper around `commit --amend` and just prevent the editor from
popping up by passing `--message`.