perf timechart: Conditionally update start_time on fork

We don't need to overwrite current task start_time on fork, so update it
only if it's zero.

Signed-off-by: Stanislav Fomichev <stfomichev@yandex-team.ru>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Link: http://lkml.kernel.org/n/1404835423-23098-4-git-send-email-stfomichev@yandex-team.ru
Signed-off-by: Jiri Olsa <jolsa@kernel.org>

tools/perf/builtin-timechart.c

@@ -242,7 +242,7 @@ static void pid_fork(struct timechart *tchart, int pid, int ppid, u64 timestamp)
 		pid_set_comm(tchart, pid, pp->current->comm);
 
 	p->start_time = timestamp;
-	if (p->current) {
+	if (p->current && !p->current->start_time) {
 		p->current->start_time = timestamp;
 		p->current->state_since = timestamp;
 	}