Join options to use numeric user and group id

* join NumericUid and NumericGid in Tar struct
* name the new option NumericUidGid

tar.go

@@ -17,11 +17,8 @@ func init() {
 }
 
 type Tar struct {
-	// If true, leave only numeric user id
-	NumericUid bool
-
-	// If true, leave only numeric group id
-	NumericGid bool
+	// If true, leave only numeric user and group id
+	NumericUidGid bool
 
 	// If true, errors encountered during reading or writing
 	// a file within an archive will be logged and the
@@ -87,10 +84,8 @@ func (t Tar) writeFileToArchive(ctx context.Context, tw *tar.Writer, file File)
 		return fmt.Errorf("file %s: creating header: %w", file.NameInArchive, err)
 	}
 	hdr.Name = file.NameInArchive // complete path, since FileInfoHeader() only has base name
-	if t.NumericUid {
+	if t.NumericUidGid {
 		hdr.Uname = ""
-	}
-	if t.NumericGid {
 		hdr.Gname = ""
 	}