t1503: Fix arithmetic expansion syntax error when using dash

t1503: Fix arithmetic expansion syntax error when using dash

On systems which have dash as /bin/sh, such as Ubuntu, the final
test (master@{n} for various n) fails with a syntax error while
processing an arithmetic expansion. The syntax error is caused by
using a bare name ('N') as a variable reference in the expression.

In order to avoid the syntax error, we spell the variable reference
as '$N' rather than simply 'N'.

Signed-off-by: Ramsay Jones <ramsay@ramsay1.demon.co.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>