Fix autocompletion of readline with directory names starting with '@' (bug #60640)

Don't insert quoting character before a directory name that begins with '@'.
* input.cc (input_system::initialize): Remove '@' from list of characters
passed to set_filename_quote_characters().

Author: Rik

libinterp/corefcn/input.cc

@@ -437,7 +437,7 @@ input_system::initialize (bool line_editing)
 
   command_editor::set_basic_quote_characters (R"(")");
 
-  command_editor::set_filename_quote_characters (" \t\n\\\"'@<>=;|&()#$`?*[!:{");
+  command_editor::set_filename_quote_characters (" \t\n\\\"'<>=;|&()#$`?*[!:{");
 
   command_editor::set_completer_quote_characters (R"('")");