perf script: Fix --reltime with --time

My earlier patch to just enable --reltime with --time was a little too
optimistic.  The --time parsing would accept absolute time, which is
very confusing to the user.

Support relative time in --time parsing too. This only works with recent
perf record that records the first sample time. Otherwise we error out.

Fixes: 3714437d ("perf script: Allow --time with --reltime")
Signed-off-by: Andi Kleen <ak@linux.intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Link: http://lore.kernel.org/lkml/20191011182140.8353-1-andi@firstfloor.org


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