func NewGitHubDeferredChangedFilesProvider(client githubClient, org, repo string, num int) ChangedFilesProvider {
	var changedFiles []string
	return func() ([]string, error) {
		// Fetch the changed files from github at most once.
		if changedFiles == nil {
			changes, err := client.GetPullRequestChanges(org, repo, num)
			if err != nil {
				return nil, fmt.Errorf("error getting pull request changes: %v", err)
			}
			for _, change := range changes {
				changedFiles = append(changedFiles, change.Filename)
			}
		}
		return changedFiles, nil
	}
}
func NewGitHubDeferredChangedFilesProvider(client githubClient, org, repo string, num int) ChangedFilesProvider {
	var changedFiles []string
	return func() ([]string, error) {
		// Fetch the changed files from github at most once.
		if changedFiles == nil {
			changes, err := client.GetPullRequestChanges(org, repo, num)
			if err != nil {
				return nil, fmt.Errorf("error getting pull request changes: %v", err)
			}
			for _, change := range changes {
				changedFiles = append(changedFiles, change.Filename)
			}
		}
		return changedFiles, nil
	}
}
// NewGitHubDeferredChangedFilesProvider uses a closure to lazily retrieve the file changes only if they are needed.
// We only have to fetch the changes if there is at least one RunIfChanged job that is not being force run (due to
// a `/retest` after a failure or because it is explicitly triggered with `/test foo`).