rebase: really just passthru the `git am` options

rebase: really just passthru the `git am` options

Currently, we parse the options intended for `git am` as if we wanted to
handle them in `git rebase`, and then reconstruct them painstakingly to
define the `git_am_opt` variable.

However, there is a much better way (that I was unaware of, at the time
when I mentored Pratik to implement these options): OPT_PASSTHRU_ARGV.
It is intended for exactly this use case, where command-line options
want to be parsed into a separate `argv_array`.

Let's use this feature.

Incidentally, this also allows us to address a bug discovered by Phillip
Wood, where the built-in rebase failed to understand that the `-C`
option takes an optional argument.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>