git-am: fix shell quoting

Noticed by Stephan Beyer; the new test is mine.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-am.sh

@@ -38,7 +38,7 @@ sq () {
 	for sqarg
 	do
 		printf "%s" "$sqarg" |
-		sed -e 's/'\''/'\''\'\'''\''/g' -e 's/.*/ '\''&'\''/'
+		sed -e 's/'\''/'\''\\'\'''\''/g' -e 's/.*/ '\''&'\''/'
 	done
 }