func byRepoAndNumber(prs []PullRequest) map[string]PullRequest {
	m := make(map[string]PullRequest)
	for _, pr := range prs {
		key := prKey(&pr)
		m[key] = pr
	}
	return m
}
func byRepoAndNumber(prs []PullRequest) map[string]PullRequest {
	m := make(map[string]PullRequest)
	for _, pr := range prs {
		key := prKey(&pr)
		m[key] = pr
	}
	return m
}
// org/repo#number -> pr