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.

modules/git/repo_blob_gogit.go

@@ -9,8 +9,8 @@ import (
 	"github.com/go-git/go-git/v5/plumbing"
 )
 
-func (repo *Repository) getBlob(id SHA1) (*Blob, error) {
-	encodedObj, err := repo.gogitRepo.Storer.EncodedObject(plumbing.AnyObject, id)
+func (repo *Repository) getBlob(id ObjectID) (*Blob, error) {
+	encodedObj, err := repo.gogitRepo.Storer.EncodedObject(plumbing.AnyObject, plumbing.Hash(id.RawValue()))
 	if err != nil {
 		return nil, ErrNotExist{id.String(), ""}
 	}