Migrate the fix for 6892 into reader itself

No functional change here; this migrates the fix ensuring that history
items are available in the builtin interactive read command into the
reader itself, in preparation for removing reader_get_history().

src/builtin_read.cpp

@@ -220,9 +220,8 @@ static int read_interactive(parser_t &parser, wcstring &buff, int nchars, bool s
 
     conf.in = in;
 
-    // Don't keep history.
+    // Keep in-memory history only.
     reader_push(parser, wcstring{}, std::move(conf));
-    reader_get_history()->resolve_pending();
 
     reader_set_buffer(commandline, std::wcslen(commandline));
     scoped_push<bool> interactive{&parser.libdata().is_interactive, true};

src/reader.cpp

@@ -2568,6 +2568,7 @@ static std::shared_ptr<reader_data_t> reader_push_ret(parser_t &parser,
                                                       const wcstring &history_name,
                                                       reader_config_t &&conf) {
     std::shared_ptr<history_t> hist = history_t::with_name(history_name);
+    hist->resolve_pending(); // see #6892
     auto data = std::make_shared<reader_data_t>(parser.shared(), hist, std::move(conf));
     reader_data_stack.push_back(data);
     data->command_line_changed(&data->command_line);

src/reader.h

@@ -240,8 +240,8 @@ struct reader_config_t {
     int in{0};
 };
 
-/// Push a new reader environment controlled by \p conf.
-/// If \p history_name is not empty, then use the history with that name.
+/// Push a new reader environment controlled by \p conf, using the given history name.
+/// If \p history_name is empty, then save history in-memory only; do not write it to disk.
 void reader_push(parser_t &parser, const wcstring &history_name, reader_config_t &&conf);
 
 /// Return to previous reader environment.