Currently many perf commands annotate/evlist/report/script/lock etc all
support "-i" option to chose a specific perf data, and all of them
create a local "input_name" to save the file name for that perf data.

Since most of these commands need it, we can add a global variable for
it, also it can some other benefits:

1. When calling script browser inside hists/annotation browser, it needs
to know the perf data file name to run that script.

2. For further feature like runtime switching to another perf data file,
this variable can also help.
Signed-off-by: Feng Tang <feng.tang@intel.com>
Cc: Andi Kleen <andi@firstfloor.org>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1351569369-26732-2-git-send-email-feng.tang@intel.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>