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>

t/lib-git-p4.sh

@@ -106,7 +106,7 @@ start_p4d () {
 		fi
 		sleep 1
 		nr_tries_left=$(($nr_tries_left - 1))
-	done &
+	done 2>/dev/null 4>&2 &
 	watchdog_pid=$!
 
 	ready=