rebase--am: use --cherry-pick instead of --ignore-if-in-upstream

rebase--am: use --cherry-pick instead of --ignore-if-in-upstream

When using `git format-patch --ignore-if-in-upstream` we are only
allowed to give a single revision range.  In the next commit we will
want to add an additional exclusion revision in order to handle fork
points correctly, so convert `git-rebase--am` to use a symmetric
difference with `--cherry-pick --right-only`.

This does not change the result of the format-patch invocation, just how
we spell the arguments.

Signed-off-by: John Keeping <john@keeping.me.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>