refactor: Drop offset in favor of ignoring the first index

cmd/stbak/cmd/archive.go

@@ -173,7 +173,6 @@ var archiveCmd = &cobra.Command{
 			int(lastIndexedBlock),
 			viper.GetBool(overwriteFlag),
 
-			0,
 			func(hdr *tar.Header, i int) error {
 				if len(hdrs) <= i {
 					return errMissingTarHeader

cmd/stbak/cmd/recovery_index.go

@@ -77,7 +77,6 @@ var recoveryIndexCmd = &cobra.Command{
 			viper.GetInt(blockFlag),
 			viper.GetBool(overwriteFlag),
 
-			0,
 			func(hdr *tar.Header, i int) error {
 				return encryption.DecryptHeader(hdr, viper.GetString(encryptionFlag), identity)
 			},

cmd/stbak/cmd/update.go

@@ -119,13 +119,16 @@ var updateCmd = &cobra.Command{
 			int(lastIndexedBlock),
 			false,
 
-			1,
 			func(hdr *tar.Header, i int) error {
-				if len(hdrs) <= i {
+				if i == 0 {
+					return nil
+				}
+
+				if len(hdrs) <= i-1 {
 					return errMissingTarHeader
 				}
 
-				*hdr = *hdrs[i]
+				*hdr = *hdrs[i-1]
 
 				return nil
 			},