bugzilla: improve identifierNotForPull error

The `release-controller` is encountering an error for an invalid pull
identifier for a github link that is not a pull. This PR imporves
identification for non-pull github links to prevent that.

prow/bugzilla/client.go

@@ -968,12 +968,12 @@ func IdentifierForPull(org, repo string, num int) string {
 
 func PullFromIdentifier(identifier string) (org, repo string, num int, err error) {
 	parts := strings.Split(identifier, "/")
+	if len(parts) >= 3 && parts[2] != "pull" {
+		return "", "", 0, &identifierNotForPull{identifier: identifier}
+	}
 	if len(parts) != 4 && !(len(parts) == 5 && (parts[4] == "" || parts[4] == "files")) && !(len(parts) == 6 && (parts[4] == "files" && parts[5] == "")) {
 		return "", "", 0, fmt.Errorf("invalid pull identifier with %d parts: %q", len(parts), identifier)
 	}
-	if parts[2] != "pull" {
-		return "", "", 0, &identifierNotForPull{identifier: identifier}
-	}
 	number, err := strconv.Atoi(parts[3])
 	if err != nil {
 		return "", "", 0, fmt.Errorf("invalid pull identifier: could not parse %s as number: %w", parts[3], err)