benchstat: don't print geomean of just one benchmark
benchstat: don't print geomean of just one benchmark

If there's just one benchmark, the geomean will be the same as the
benchmark result. Omit it.

Change-Id: Id165fa127aa8615015511d6a1da6e1be48b85539
Reviewed-on: https://go-review.googlesource.com/38093
Reviewed-by: Quentin Smith <d6b8e48afb2534b213e391cab43016505747a234@golang.org>

golang/perf,golang/perf