builtin rebase: support `-C` and `--whitespace=<type>`

builtin rebase: support `-C` and `--whitespace=<type>`

This commit converts more code from the shell script version to the
builtin rebase. In this instance, we just have to be careful to
keep support for passing multiple `--whitespace` options, as the
shell script version does so, too.

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