No duplicate s-o-b lines

Luca reported that if a patch has:

Signed-off-by: Ed Example <ed@example.com>
Link: http://www.example.com/

Then pwcli (and git am -s) add a second s-o-b line. I guess it could be handled so that check if there's already s-o-b don't use '-s' switch with git-am.
