perf hists: Support multiple sort keys in a hierarchy level

This implements having multiple sort keys in a single hierarchy level.
Originally only single sort key is supported for each level, but now
using the group syntax with '{ }', it can set more than one sort key in
one level.  Note that now it needs to quote in order to prevent shell
interpretation.

For example:

  $ perf report --hierarchy -s '{comm,dso},sym'
  ...
  #       Overhead  Command / Shared Object / Symbol
  # ..............  ..........................................
  #
      48.67%        swapper          [kernel.vmlinux]
         34.42%        [k] intel_idle
          1.30%        [k] __tick_nohz_idle_enter
          1.03%        [k] cpuidle_reflect
       8.87%        firefox          libpthread-2.22.so
          6.60%        [.] __GI___libc_recvmsg
          1.18%        [.] pthread_cond_signal@@GLIBC_2.3.2
          1.09%        [.] 0x000000000000ff4b
       6.11%        Xorg             libc-2.22.so
          5.27%        [.] __memcpy_sse2_unaligned

In the above example, the command name and the shared object name are
shown on the same line but the symbol name is on the different line.
Since the first two are grouped by '{}', they are in the same level.

Suggested-and-Tested=by: Arnaldo Carvalho de Melo <acme@kernel.org>
Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/r/1457361308-514-4-git-send-email-namhyung@kernel.org


Signed-off-by: Ingo Molnar <mingo@kernel.org>