perf stat: Introduce --repeat forever

The following patch causes 'perf stat --repeat 0' to be interpreted as
'forever', displaying the stats for every run.

We act as if a single run was asked, and reset the stats in each
iteration. In this mode SIGINT is passed to perf to be able to stop the
loop with Ctrl+C.

Signed-off-by: Frederik Deweerdt <frederik.deweerdt@gmail.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/20130301180227.GA24385@ks398093.ip-192-95-24.net
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
diff --git a/tools/perf/Documentation/perf-stat.txt b/tools/perf/Documentation/perf-stat.txt
index faf4f4f..23e587a 100644
--- a/tools/perf/Documentation/perf-stat.txt
+++ b/tools/perf/Documentation/perf-stat.txt
@@ -52,7 +52,7 @@
 
 -r::
 --repeat=<n>::
-	repeat command and print average + stddev (max: 100)
+	repeat command and print average + stddev (max: 100). 0 means forever.
 
 -B::
 --big-num::