go.tools/dashboard: update existing results when builder retries a commit
go.tools/dashboard: update existing results when builder retries a commit

LGTM=adg
R=adg
CC=golang-codereviews
https://codereview.appspot.com/96360047

t-yuki/go.tools.old,t-yuki/go.tools.old,t-yuki/go.tools.old