Adjust object format interface (#28469)

- Remove `ObjectFormatID`
- Remove function `ObjectFormatFromID`.
- Use `Sha1ObjectFormat` directly but not a pointer because it's an
empty struct.
- Store `ObjectFormatName` in `repository` struct

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 == repo.objectFormat.Empty().String() {
+	if base == repo.objectFormat.EmptyObjectID().String() {
 		cmd.AddDynamicArguments(head)
 	} else {
 		cmd.AddDynamicArguments(base, head)