git p4 test: disable '-x' tracing in the p4d watchdog loop

According to the comments in 't/lib-git-p4.sh', sometimes 'p4d' seems to hang, and to deal with that 'start_p4d' starts a watchdog process to kill it after a long-enough timeout ($P4D_TIMEOUT, defaults to 300s). This watchdog process is implemented as a background subshell loop iterating once every second until the timeout expires, producing a few lines of trace output on each iteration when the test script is run with '-x' tracing enabled. The watchdog loop's trace gets intermixed with the real test output and trace, and makes that harder to read. Send the trace output of this loop to /dev/null to avoid polluting the real test output. Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>