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_index.go

@@ -101,7 +101,7 @@ func (repo *Repository) RemoveFilesFromIndex(filenames ...string) error {
 	for _, file := range filenames {
 		if file != "" {
 			buffer.WriteString("0 ")
-			buffer.WriteString(repo.objectFormat.Empty().String())
+			buffer.WriteString(repo.objectFormat.EmptyObjectID().String())
 			buffer.WriteByte('\t')
 			buffer.WriteString(file)
 			buffer.WriteByte('\000')