Add success callback to the backend

cmd/restic/global.go

@@ -439,9 +439,13 @@ func OpenRepository(opts GlobalOptions) (*repository.Repository, error) {
 		return nil, err
 	}
 
-	be = backend.NewRetryBackend(be, 10, func(msg string, err error, d time.Duration) {
+	report := func(msg string, err error, d time.Duration) {
 		Warnf("%v returned error, retrying after %v: %v\n", msg, d, err)
-	})
+	}
+	success := func(msg string, retries int) {
+		Warnf("%v operation successful after %d retries\n", msg, retries)
+	}
+	be = backend.NewRetryBackend(be, 10, report, success)
 
 	// wrap backend if a test specified a hook
 	if opts.backendTestHook != nil {