download: support -x when cherry-picking

This is a pretty common option for people to want too use, so include
it as a pass-thru option when cherry-picking.

Bug: https://crbug.com/gerrit/9418
Change-Id: I2a24c1ed7544541719caa4d3c0574347a151a1b0
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/259853


Reviewed-by: David Pursehouse <dpursehouse@collab.net>
Tested-by: Mike Frysinger <vapier@google.com>