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_compare.go

@@ -284,7 +284,7 @@ func (repo *Repository) GetPatch(base, head string, w io.Writer) error {
 // If base is the SHA of an empty tree (EmptyTreeSHA), it returns the files changes from the initial commit to the head commit
 func (repo *Repository) GetFilesChangedBetween(base, head string) ([]string, error) {
 	cmd := NewCommand(repo.Ctx, "diff-tree", "--name-only", "--root", "--no-commit-id", "-r", "-z")
-	if base == EmptySHA {
+	if base == repo.objectFormat.Empty().String() {
 		cmd.AddDynamicArguments(head)
 	} else {
 		cmd.AddDynamicArguments(base, head)