am: POSIX portability fix

POSIX allows echo without flag to interpret specials such as \n, and we
tried to make things portable by using printf instead where it matters.
Recently added code to "git am" had unprotected "echo", which was caught
by t4014 and Rémi Vanicat.

This should fix it.

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

git-am.sh

@@ -302,7 +302,7 @@ do
 		SUBJECT="$(sed -n '/^Subject/ s/Subject: //p' "$dotest/info")"
 		case "$keep_subject" in -k)  SUBJECT="[PATCH] $SUBJECT" ;; esac
 
-		(echo "$SUBJECT" ; echo ; cat "$dotest/msg") |
+		(printf '%s\n\n' "$SUBJECT"; cat "$dotest/msg") |
 			git stripspace > "$dotest/msg-clean"
 		;;
 	esac