perf diff: Display data file info ahead of the diff output

Data files are referenced through the index of the file on the command
line. Adding list of data files for each index to ease up navigation for
user.

It's displayed only if in verbose mode.

Signed-off-by: Jiri Olsa <jolsa@redhat.com>
Reviewed-by: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Corey Ashford <cjashfor@linux.vnet.ibm.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: http://lkml.kernel.org/n/tip-dfjxa6n116ughjjxohpkuvi8@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>