[clang-tidy] Remove redundant c_str

Found with readability-redundant-string-cstr

src/reader.cpp

@@ -2754,7 +2754,7 @@ void reader_data_t::handle_readline_command(readline_cmd_t c, readline_loop_stat
                 if (abbreviation_expanded) {
                     // It's our reponsibility to rehighlight and repaint. But everything we do
                     // below triggers a repaint.
-                    command_test_result = test_func(parser(), el->text.c_str());
+                    command_test_result = test_func(parser(), el->text);
 
                     // If the command is OK, then we're going to execute it. We still want to do
                     // syntax highlighting, but a synchronous variant that performs no I/O, so