perf: store subsection results in "test-results/$GIT_PERF_SUBSECTION/"

perf: store subsection results in "test-results/$GIT_PERF_SUBSECTION/"

When tests are run for a subsection defined in a config file, it is
better if the results for the current subsection are not overwritting
the results of a previous subsection.

So let's store the results for a subsection in a subdirectory of
"test-results/" with the subsection name.

The aggregate.perl, when it is run for a subsection, should then
aggregate the results found in "test-results/$GIT_PERF_SUBSECTION/".

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