t/perf: add fallback for pre-bin-wrappers versions of git

It's tempting to say: ./run v1.0.0 HEAD to see how we've sped up Git over the years. Unfortunately, this doesn't quite work because versions of Git prior to v1.7.0 lack bin-wrappers, so our "run" script doesn't correctly put them in the PATH. Worse, it means we silently find whatever other "git" is in the PATH, and produce test results that have no bearing on what we asked for. Let's fallback to the main git directory when bin-wrappers isn't present. Many modern perf scripts won't run with such an antique version of Git, of course, but at least those failures are detected and reported (and you're free to write a limited perf script that works across many versions). Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>