Abstract hash function usage (#28138)

Refactor Hash interfaces and centralize hash function. This will allow
easier introduction of different hash function later on.

This forms the "no-op" part of the SHA256 enablement patch.

services/migrations/gitea_uploader.go

@@ -892,7 +892,8 @@ func (g *GiteaLocalUploader) CreateReviews(reviews ...*base.Review) error {
 				comment.UpdatedAt = comment.CreatedAt
 			}
 
-			if !git.IsValidSHAPattern(comment.CommitID) {
+			objectFormat, _ := g.gitRepo.GetObjectFormat()
+			if !objectFormat.IsValid(comment.CommitID) {
 				log.Warn("Invalid comment CommitID[%s] on comment[%d] in PR #%d of %s/%s replaced with %s", comment.CommitID, pr.Index, g.repoOwner, g.repoName, headCommitID)
 				comment.CommitID = headCommitID
 			}