It is a good idea to error out early upon seeing, say, `-Cbad`, rather
than starting the rebase only to have the `--am` backend complain later.

Let's do this.

The only options accepting parameters which we pass through to `git am`
(which may, or may not, forward them to `git apply`) are `-C` and
`--whitespace`. The other options we pass through do not accept
parameters, so we do not have to validate them here.
Signed-off-by: NJohannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>