perf/aggregate: add --subsection option

This makes it easier to use the aggregate script
on the command line, to get results from
subsections.

Previously setting GIT_PERF_SUBSECTION was needed
for this purpose.

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>