It would be nice if we can use a hotkey in perf top browser to select a
event for sorting.

For example:

  perf top --group -e cycles,instructions,cache-misses

  Samples
                  Overhead  Shared Object             Symbol
    40.03%  45.71%   0.03%  div                       [.] main
    20.46%  14.67%   0.21%  libc-2.27.so              [.] __random_r
    20.01%  19.54%   0.02%  libc-2.27.so              [.] __random
     9.68%  10.68%   0.00%  div                       [.] compute_flag
     4.32%   4.70%   0.00%  libc-2.27.so              [.] rand
     3.84%   3.43%   0.00%  div                       [.] rand@plt
     0.05%   0.05%   2.33%  libc-2.27.so              [.] __strcmp_sse2_unaligned
     0.04%   0.08%   2.43%  perf                      [.] perf_hpp__is_dynamic_en
     0.04%   0.02%   6.64%  perf                      [.] rb_next
     0.04%   0.01%   3.87%  perf                      [.] dso__find_symbol
     0.04%   0.04%   1.77%  perf                      [.] sort__dso_cmp

When user press hotkey '2' (event index, starting from 0), it indicates
to sort output by the third event in group (cache-misses).

  Samples
                  Overhead  Shared Object               Symbol
     4.07%   1.28%   6.68%  perf                        [.] rb_next
     3.57%   3.98%   4.11%  perf                        [.] __hists__insert_output
     3.67%  11.24%   3.60%  perf                        [.] perf_hpp__is_dynamic_e
     3.67%   3.20%   3.20%  perf                        [.] hpp__sort_overhead
     0.81%   0.06%   3.01%  perf                        [.] dso__find_symbol
     1.62%   5.47%   2.51%  perf                        [.] hists__match
     2.70%   1.86%   2.47%  libc-2.27.so                [.] _int_malloc
     0.19%   0.00%   2.29%  [kernel]                    [k] copy_page
     0.41%   0.32%   1.98%  perf                        [.] hists__decay_entries
     1.84%   3.67%   1.68%  perf                        [.] sort__dso_cmp
     0.16%   0.00%   1.63%  [kernel]                    [k] clear_page_erms

Now the output is sorted by cache-misses.

 v2:
 ---
 Zero the history if hotkey is pressed.
Signed-off-by: Jin Yao <yao.jin@linux.intel.com>
Suggested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Jin Yao <yao.jin@intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Kan Liang <kan.liang@linux.intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lore.kernel.org/lkml/20200324220711.6025-2-yao.jin@linux.intel.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>