perf script: Don't force less for non tty output with --xed

--xed currently forces less. When piping the output to other scripts
this can waste a lot of CPU time because less is rather slow.
I've seen it using up a full core on its own in a pipeline.
Only force less when the output is actually a terminal.

Signed-off-by: Andi Kleen <ak@linux.intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Link: http://lore.kernel.org/lkml/20200522020914.527564-1-andi@firstfloor.org


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