t2300: use documented technique to invoke git-sh-setup

This is needed to allow the test suite to run against a standard
install bin directory instead of GIT_EXEC_PATH.

Signed-off-by: Matthew Ogilvie <mmogilvi_git@miniinfo.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t2300-cd-to-toplevel.sh

@@ -8,7 +8,7 @@ test_cd_to_toplevel () {
 	test_expect_success $3 "$2" '
 		(
 			cd '"'$1'"' &&
-			. git-sh-setup &&
+			. "$(git --exec-path)"/git-sh-setup &&
 			cd_to_toplevel &&
 			[ "$(pwd -P)" = "$TOPLEVEL" ]
 		)