test-lib tests: avoid subshell for "test_cmp" for readability

The use of a sub-shell for running the test_cmp of stdout/stderr for
the test author was introduced in this form in 565b6fa87b (tests:
refactor mechanics of testing in a sub test-lib, 2012-12-16), but from
looking at the history that seemed to have diligently copied my
original ad-hoc implementation in 7b90511970 (t/t0000-basic.sh: Run
the passing TODO test inside its own test-lib, 2010-08-19).

There's no reason to use a subshell here, we try to avoid it in
general. It also improves readability, if the test fails we print out
the relative path in the trash directory that needs to be looked
at.

Before that was mostly obscured, since the "write_sub_test_lib_test"
will pick the directory for you from the test name.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/lib-subtest.sh

@@ -79,22 +79,16 @@ run_sub_test_lib_test_err () {
 
 check_sub_test_lib_test () {
 	name="$1" # stdin is the expected output from the test
-	(
-		cd "$name" &&
-		test_must_be_empty err &&
-		sed -e 's/^> //' -e 's/Z$//' >expect &&
-		test_cmp expect out
-	)
+	test_must_be_empty "$name"/err &&
+	sed -e 's/^> //' -e 's/Z$//' >"$name"/expect &&
+	test_cmp "$name/"expect "$name"/out
 }
 
 check_sub_test_lib_test_err () {
 	name="$1" # stdin is the expected output from the test
 	# expected error output is in descriptor 3
-	(
-		cd "$name" &&
-		sed -e 's/^> //' -e 's/Z$//' >expect.out &&
-		test_cmp expect.out out &&
-		sed -e 's/^> //' -e 's/Z$//' <&3 >expect.err &&
-		test_cmp expect.err err
-	)
+	sed -e 's/^> //' -e 's/Z$//' >"$name"/expect.out &&
+	test_cmp "$name"/expect.out "$name"/out &&
+	sed -e 's/^> //' -e 's/Z$//' <&3 >"$name"/expect.err &&
+	test_cmp "$name"/expect.err "$name"/err
 }