misc/dashboard/codereview: only accept "NOT LGTM" on the first line of a message.

Too many people quote entire emails and put their reply at the top ("top posting"),
so we shouldn't recognise review commands anywhere in the review text.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/6815048

misc/dashboard/codereview/dashboard/cl.go

@@ -389,7 +389,7 @@ func updateCL(c appengine.Context, n string) error {
 			lgtm[s] = true
 			delete(notLGTM, s) // "LGTM" overrules previous "NOT LGTM"
 		}
-		if strings.Contains(msg.Text, "NOT LGTM") {
+		if strings.Contains(line, "NOT LGTM") {
 			notLGTM[s] = true
 			delete(lgtm, s) // "NOT LGTM" overrules previous "LGTM"
 		}