cmd/bench: add subrepo directories to run benchmarks on

For golang/go#53538

Change-Id: I0eb1d2f00ca1977bdd909e0a4bfe4367d388ba16
Reviewed-on: https://go-review.googlesource.com/c/benchmarks/+/453502
Reviewed-by: Michael Pratt <mpratt@google.com>
Run-TryBot: Dylan Le <dungtuanle@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
3 files changed