perf diff: Change diff command to work over multiple data files

Adding diff command the flexibility to specify multiple data files on
input. If not input file is given the standard behaviour stands and diff
inspects 'perf.data' and 'perf.data.old' files.

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-8j3xer54ltvs76t0fh01gcvu@git.kernel.org


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