t/perf: add 'GIT_PERF_USE_SCALAR' run option

t/perf: add 'GIT_PERF_USE_SCALAR' run option

Add a 'GIT_PERF_USE_SCALAR' environment variable (and corresponding perf
config 'useScalar') to register a repository created with any of:

* test_perf_fresh_repo
* test_perf_default_repo
* test_perf_large_repo

as a Scalar enlistment. This is intended to allow a developer to test the
impact of Scalar on already-defined performance scenarios.

Suggested-by: Derrick Stolee <derrickstolee@github.com>
Signed-off-by: Victoria Dye <vdye@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>