x/tools/dashboard/app: prevent builders from nuking other results

This fixes the issue where a builder would occasionally wipe out
the results from another builder.

LGTM=bradfitz
R=bradfitz
CC=golang-codereviews
https://golang.org/cl/172150043

dashboard/app/build/build.go

@@ -154,7 +154,7 @@ func (com *Commit) AddResult(c appengine.Context, r *Result) error {
 	var resultExists bool
 	for i, s := range com.ResultData {
 		// if there already exists result data for this builder at com, overwrite it.
-		if strings.Contains(s, r.Builder) {
+		if strings.HasPrefix(s, r.Builder+"|") {
 			resultExists = true
 			com.ResultData[i] = r.Data()
 		}