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>
1 file changed