t/perf: add Scalar performance tests

t/perf: add Scalar performance tests

Create 'p9210-scalar.sh' for testing Scalar performance and comparing
performance of Git operations in Scalar registrations and standard
repositories. Example results:

Test                                                   this tree
------------------------------------------------------------------------
9210.2: scalar clone                                   14.82(18.00+3.63)
9210.3: git clone                                      26.15(36.67+6.90)
9210.4: git status (scalar)                            0.04(0.01+0.01)
9210.5: git status (non-scalar)                        0.10(0.02+0.11)
9210.6: test_commit --append --no-tag A (scalar)       0.08(0.02+0.03)
9210.7: test_commit --append --no-tag A (non-scalar)   0.13(0.03+0.11)

Helped-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Victoria Dye <vdye@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>