perf/aggregate: implement codespeed JSON output

perf/aggregate: implement codespeed JSON output

Codespeed (https://github.com/tobami/codespeed/) is an open source
project that can be used to track how some software performs over
time. It stores performance test results in a database and can show
nice graphs and charts on a web interface.

As it can be interesting to use Codespeed to see how Git performance
evolves over time and releases, let's implement a Codespeed output
in "perf/aggregate.perl".

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