t/perf: add iteration setup mechanism to perf-lib

t/perf: add iteration setup mechanism to perf-lib

Tests that affect the repo in stateful ways are easier to write if we
can run setup steps outside of the measured portion of perf iteration.

This change adds a "--setup 'setup-script'" parameter to test_perf. To
make invocations easier to understand, I also moved the prerequisites to
a new --prereq parameter.

The setup facility will be used in the upcoming perf tests for batch
mode, but it already helps in some existing tests, like t5302 and t7820.

Signed-off-by: Neeraj Singh <neerajsi@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>