// GetFirstCommit returns the first commit that is included in the review
func (r DifferentialReview) GetFirstCommit(repo repository.Repo) string {
	var commits []string
	for _, hashPair := range r.Hashes {
		// We only care about the hashes for commits, which have exactly two
		// elements, the first of which is "gtcm".
		if len(hashPair) == 2 && hashPair[0] == commitHashType {
			commits = append(commits, hashPair[1])
		}
	}
	var commitTimestamps []int
	commitsByTimestamp := make(map[int]string)
	for _, commit := range commits {
		if _, err := repo.GetLastParent(commit); err == nil {
			timeString, err1 := repo.GetCommitTime(commit)
			timestamp, err2 := strconv.Atoi(timeString)
			if err1 == nil && err2 == nil {
				commitTimestamps = append(commitTimestamps, timestamp)
				// If there are multiple, equally old commits, then the last one wins.
				commitsByTimestamp[timestamp] = commit
			}
		}
	}
	if len(commitTimestamps) == 0 {
		return ""
	}
	sort.Ints(commitTimestamps)
	revision := commitsByTimestamp[commitTimestamps[0]]
	return revision
}