perf timechart: Add option to limit number of tasks

Add -n option to specify min. number of tasks to print.

Signed-off-by: Stanislav Fomichev <stfomichev@yandex-team.ru>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1383323151-19810-3-git-send-email-stfomichev@yandex-team.ru


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>