download: support --ff when cherry-picking

The git cherry-pick already supports this, so plumb the existing repo
option down.  Otherwise it's confusing when people use -c --ff and it
doesn't use that behavior.

Change-Id: Id68932ffa09204bb30b92a21aff185c00394a520
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/259852


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