t5570-git-daemon: don't check the stderr of a subshell

The test 'no-op fetch without "-v" is quiet' in 't5570-git-daemon.sh'
fails when the test script is run with '-x' tracing (and using a shell
other than a Bash version supporting BASH_XTRACEFD).  The reason for
the failure is that the test checks the emptiness of a subshell's
stderr, which includes the trace of commands executed in that subshell
as well, throwing off the emptiness check.

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

After this change t5570 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/t5570-git-daemon.sh

@@ -50,7 +50,7 @@ test_expect_success 'no-op fetch -v stderr is as expected' '
 '
 
 test_expect_success 'no-op fetch without "-v" is quiet' '
-	(cd clone && git fetch) 2>stderr &&
+	(cd clone && git fetch 2>../stderr) &&
 	! test -s stderr
 '