fix how fish behaves when FISH_HISTORY is set

Without this change setting `FISH_HISTORY` causes interactive input to
no longer provide autosuggestions, completions, etc.

Fixes #4234

src/reader.cpp

@@ -1995,6 +1995,11 @@ static parser_test_error_bits_t default_test(const wchar_t *b) {
     return 0;
 }
 
+void reader_change_history(const wchar_t *name) {
+    data->history->save();
+    data->history = &history_t::history_with_name(name);
+}
+
 void reader_push(const wchar_t *name) {
     reader_data_t *n = new reader_data_t();