perf report: Fix comm sorting

Since we can (and do) change comm strings during the collection
phase, we cannot actually sort on them to build the histogram.
Therefore add an (optional) third sorting phase to collapse the
histrogram.

Comm sorting now builds the histrogram on threads and then in
the collapse phase collects all threads with the same comm.

This collapsed histogram is then reversed and sorted on events.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Corey Ashford <cjashfor@linux.vnet.ibm.com>
Cc: Marcelo Tosatti <mtosatti@redhat.com>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: John Kacur <jkacur@redhat.com>
LKML-Reference: <new-submission>
Signed-off-by: Ingo Molnar <mingo@elte.hu>