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>

t/t1503-rev-parse-verify.sh

@@ -106,8 +106,8 @@ test_expect_success 'use --default' '
 
 test_expect_success 'master@{n} for various n' '
 	N=$(git reflog | wc -l) &&
-	Nm1=$((N-1)) &&
-	Np1=$((N+1)) &&
+	Nm1=$(($N-1)) &&
+	Np1=$(($N+1)) &&
 	git rev-parse --verify master@{0} &&
 	git rev-parse --verify master@{1} &&
 	git rev-parse --verify master@{$Nm1} &&