rebase -p: error out if --signoff is given

rebase --preserve-merges does not support --signoff so error out
rather than just silently ignoring it so that the user knows the
commits will not be signed off.

Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-rebase.sh

@@ -472,6 +472,8 @@ fi
 
 if test -n "$signoff"
 then
+	test -n "$preserve_merges" &&
+		die "$(gettext "error: cannot combine '--signoff' with '--preserve-merges'")"
 	git_am_opt="$git_am_opt $signoff"
 	force_rebase=t
 fi