perf timechart: Use tid not pid for COMM change

A process that changes its comm field, does this on a per kernel
task struct basis. The timechart tool used, incorrectly, the pid
to track this, and should have used the tid instead...

Signed-off-by: Arjan van de Ven <arjan@linux.intel.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
CC: <stable@kernel.org>
LKML-Reference: <20100116125319.34ac3edd@infradead.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>