cmd/coordinator: populate commit label on benchmarks This was lost in one of the many refactorings of CL 38306. Updates golang/go#19871 Change-Id: I4514d4350feb1420861eae9a9dd021fee5b5ba6f Reviewed-on: https://go-review.googlesource.com/43052 Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>