perf util: Support no index time percent slice

Previously, the time percent slice needs an index to specify which one
the user wants.

It may be easier to use if the index can be omitted.  So with this
patch, for example,

perf report --stdio --time 10%/1 should be equivalent to
perf report --stdio --time 10%

Signed-off-by: Jin Yao <yao.jin@linux.intel.com>
Suggested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Reviewed-by: Jiri Olsa <jolsa@redhat.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Kan Liang <kan.liang@intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1515596433-24653-5-git-send-email-yao.jin@linux.intel.com


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