perf: fix when running with TEST_OUTPUT_DIRECTORY

perf: fix when running with TEST_OUTPUT_DIRECTORY

When the TEST_OUTPUT_DIRECTORY is defined, then all test data will be
written in that directory instead of the default directory located in
"t/". While this works as expected for our normal tests, performance
tests fail to locate and aggregate performance data because they don't
know to handle TEST_OUTPUT_DIRECTORY correctly and always look at the
default location.

Fix the issue by adding a `--results-dir` parameter to "aggregate.perl"
which identifies the directory where results are and by making the "run"
script awake of the TEST_OUTPUT_DIRECTORY variable.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>