docs: address inaccurate `--empty` default with `--exec`

docs: address inaccurate `--empty` default with `--exec`

The documentation for git-rebase(1) indicates that using the `--exec`
option will use `--empty=drop`. This is inaccurate: when `--interactive`
is not explicitly provided, `--exec` results in `--empty=keep`
behaviors.

Correctly indicate the behavior of `--exec` using `--empty=keep` when
`--interactive` is not specified.

Reported-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Brian Lyles <brianmlyles@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>