t5500-fetch-pack: don't check the stderr of a subshell

Three "missing reference" tests in 't5500-fetch-pack.sh' fail when the
test script is run with '-x' tracing (and using a shell other than a
Bash version supporting BASH_XTRACEFD).  The reason for those failures
is that the tests check a subshell's stderr, which includes the trace
of executing commands in that subshell as well, throwing off the
comparison with the expected output.

Save the stderr of 'git fetch-pack' only instead of the whole
subshell, so it remains free from tracing output.

After this change t5500 passes with '-x', even when running with
/bin/sh.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t5500-fetch-pack.sh

@@ -482,24 +482,24 @@ test_expect_success 'set up tests of missing reference' '
 test_expect_success 'test lonely missing ref' '
 	(
 		cd client &&
-		test_must_fail git fetch-pack --no-progress .. refs/heads/xyzzy
-	) >/dev/null 2>error-m &&
+		test_must_fail git fetch-pack --no-progress .. refs/heads/xyzzy 2>../error-m
+	) &&
 	test_i18ncmp expect-error error-m
 '
 
 test_expect_success 'test missing ref after existing' '
 	(
 		cd client &&
-		test_must_fail git fetch-pack --no-progress .. refs/heads/A refs/heads/xyzzy
-	) >/dev/null 2>error-em &&
+		test_must_fail git fetch-pack --no-progress .. refs/heads/A refs/heads/xyzzy 2>../error-em
+	) &&
 	test_i18ncmp expect-error error-em
 '
 
 test_expect_success 'test missing ref before existing' '
 	(
 		cd client &&
-		test_must_fail git fetch-pack --no-progress .. refs/heads/xyzzy refs/heads/A
-	) >/dev/null 2>error-me &&
+		test_must_fail git fetch-pack --no-progress .. refs/heads/xyzzy refs/heads/A 2>../error-me
+	) &&
 	test_i18ncmp expect-error error-me
 '