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.

routers/api/v1/repo/repo.go

@@ -253,6 +253,7 @@ func CreateUserRepo(ctx *context.APIContext, owner *user_model.User, opt api.Cre
 		DefaultBranch: opt.DefaultBranch,
 		TrustModel:    repo_model.ToTrustModel(opt.TrustModel),
 		IsTemplate:    opt.Template,
+		ObjectFormat:  git.ObjectFormatFromID(git.Sha1),
 	})
 	if err != nil {
 		if repo_model.IsErrRepoAlreadyExist(err) {