t5526: use $TRASH_DIRECTORY to specify the path of GIT_TRACE log file

The test 'fetch --recurse-submodules -j2 has the same output
behaviour' in 't5526-fetch-submodules.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 of that failure is the
following command:

  GIT_TRACE=$(pwd)/../trace.out git fetch <...> 2>../actual.err

because the trace of executing 'pwd' in the command substitution ends
up in 'actual.err' as well, throwing off the subsequent
'test_i18ncmp'.

Use $TRASH_DIRECTORY to specify the path of the GIT_TRACE log file
instead of $(pwd), so the command's stderr remains free from tracing
output.

After this change t5526 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/t5526-fetch-submodules.sh

@@ -85,7 +85,7 @@ test_expect_success "fetch --recurse-submodules -j2 has the same output behaviou
 	add_upstream_commit &&
 	(
 		cd downstream &&
-		GIT_TRACE=$(pwd)/../trace.out git fetch --recurse-submodules -j2 2>../actual.err
+		GIT_TRACE="$TRASH_DIRECTORY/trace.out" git fetch --recurse-submodules -j2 2>../actual.err
 	) &&
 	test_must_be_empty actual.out &&
 	test_i18ncmp expect.err actual.err &&