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>

t/perf/run

@@ -63,6 +63,9 @@ run_dirs_helper () {
 		unset GIT_TEST_INSTALLED
 	else
 		GIT_TEST_INSTALLED="$mydir/bin-wrappers"
+		# Older versions of git lacked bin-wrappers; fallback to the
+		# files in the root.
+		test -d "$GIT_TEST_INSTALLED" || GIT_TEST_INSTALLED=$mydir
 		export GIT_TEST_INSTALLED
 	fi
 	run_one_dir "$@"