perf stat: Fix adding multiple event groups

perf stat: Fix adding multiple event groups

The -M metric group parser threw away the events of earlier groups when
multiple groups were specified. Fix this here by not overwriting the
string incorrectly.

Now this works correctly:

% perf stat -M Summary,SMT --metric-only -a sleep 1

 Performance counter stats for 'system wide':

Instructions CPI CLKS         CPU_Utilization GFLOPs SMT_2T_Utilization SMT_2T_Utilization Kernel_Utilization CoreIPC CORE_CLKS
900907376.0  2.7 2398954144.0 0.1             0.0    0.2                0.2                0.1                0.4     2080822855.5

while previously it would only show the SMT metrics.

Signed-off-by: Andi Kleen <ak@linux.intel.com>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Link: http://lkml.kernel.org/r/20170914205735.18431-1-andi@firstfloor.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>