t7810: use test_expect_code() instead of hand-rolled comparison

This test manually checks the exit code of git-grep for a particular
value. In doing so, it intentionally breaks the &&-chain. Modernize the
test by taking advantage of test_expect_code() and a normal &&-chain.

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t7810-grep.sh

@@ -845,10 +845,9 @@ test_expect_success 'grep from a subdirectory to search wider area (1)' '
 test_expect_success 'grep from a subdirectory to search wider area (2)' '
 	mkdir -p s &&
 	(
-		cd s || exit 1
-		( git grep xxyyzz .. >out ; echo $? >status )
-		! test -s out &&
-		test 1 = $(cat status)
+		cd s &&
+		test_expect_code 1 git grep xxyyzz .. >out &&
+		! test -s out
 	)
 '