perf diff: Add generic order option for compute sorting

Adding option 'o' to allow sorting based on the input file number. By
default (without -o option) the output is sorted on baseline.

Also removing '+' sorting support from -c option, because it's not
needed anymore.

Signed-off-by: Jiri Olsa <jolsa@redhat.com>
Reviewed-by: Namhyung Kim <namhyung@kernel.org>
Cc: Corey Ashford <cjashfor@linux.vnet.ibm.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/n/tip-l7dvhgt0azm7yiqg3fbn4dxw@git.kernel.org


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