perf stat: Fix group lookup for metric group

The metric group code tries to find a group it added earlier in the
evlist. Fix the lookup to handle groups with partially overlaps
correctly. When a sub string match fails and we reset the match, we have
to compare the first element again.

I also renamed the find_evsel function to find_evsel_group to make its
purpose clearer.

With the earlier changes this fixes:

Before:

  % perf stat -M UPI,IPC sleep 1
  ...
         1,032,922      uops_retired.retire_slots #      1.1 UPI
         1,896,096      inst_retired.any
         1,896,096      inst_retired.any
         1,177,254      cpu_clk_unhalted.thread

After:

  % perf stat -M UPI,IPC sleep 1
  ...
        1,013,193      uops_retired.retire_slots #      1.1 UPI
           932,033      inst_retired.any
           932,033      inst_retired.any          #      0.9 IPC
         1,091,245      cpu_clk_unhalted.thread

Signed-off-by: Andi Kleen <ak@linux.intel.com>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Cc: Kan Liang <kan.liang@linux.intel.com>
Fixes: b18f3e36 ("perf stat: Support JSON metrics in perf stat")
Link: http://lkml.kernel.org/r/20190624193711.35241-4-andi@firstfloor.org


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