perf sort: Keep output fields in the same level

perf sort: Keep output fields in the same level

This is useful for hierarchy output mode where the first level is
considered as output fields.  We want them in the same level so that it
can show only the remaining groups in the hierarchy.

Before:
  $ perf report -s overhead,sample,period,comm,dso -H --stdio
  ...
  #          Overhead  Samples / Period / Command / Shared Object
  # .................  ..........................................
  #
     100.00%           4035
        100.00%           3835883066
           100.00%           perf
               99.37%           perf
                0.50%           ld-linux-x86-64.so.2
                0.06%           [unknown]
                0.04%           libc.so.6
                0.02%           libLLVM-16.so.1

After:
  $ perf report -s overhead,sample,period,comm,dso -H --stdio
  ...
  #    Overhead       Samples        Period  Command / Shared Object
  # .......................................  .......................
  #
     100.00%          4035    3835883066     perf
         99.37%          4005    3811826223     perf
          0.50%            19      19210014     ld-linux-x86-64.so.2
          0.06%             8       2367089     [unknown]
          0.04%             2       1720336     libc.so.6
          0.02%             1        759404     libLLVM-16.so.1

Acked-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20250307080829.354947-1-namhyung@kernel.org
Signed-off-by: Namhyung Kim <namhyung@kernel.org>