git-am: re-fix the diag message printing

The $FIRSTLINE variable is from the user's commit and can contain
arbitrary backslash escapes that may be (mis)interpreted when given to
"echo", depending on the implementation.  Use "printf" to work around the
issue.
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

git-am.sh

@@ -501,7 +501,7 @@ do
 	fi
 	if test $apply_status != 0
 	then
-		echo "Patch failed at $msgnum $FIRSTLINE"
+		printf 'Patch failed at %s %s\n' "$msgnum" "$FIRSTLINE"
 		stop_here_user_resolve $this
 	fi