pw-apply: make 3 way merge optional for git-am

If there are patch conflicts some maintainers prefer to get the author
to resend, or at least manually inspect the patch, so don't pass -3
as default - make it an option.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>