Merge branch 'jt/perf-updates' into maint

The t/perf performance test suite was not prepared to test not so
old versions of Git, but now it covers versions of Git that are not
so ancient.

* jt/perf-updates:
  t/perf: add fallback for pre-bin-wrappers versions of git
  t/perf: use $MODERN_GIT for all repo-copying steps
  t/perf: export variable used in other blocks