Fix file_dialog's root_subfolder on Windows

`root_prefix` either contains an empty string or the current root
including the drive letter. This means that the previous logic would
never ever match since `dir_access->get_current_dir(false)` explicitly
excludes the drive letter.

This change removes this boolean parameter so we compare paths in a way
that can match.

I've had a look at the implementations for other platforms for
`DirAccess::get_current_dir(bool include_drive)` and outside Windows
none seem to consider the `include_drive` parameter which makes me
believe that this change won't break other platforms.

Author: paul-marechal

scene/gui/file_dialog.cpp

@@ -1448,7 +1448,7 @@ void FileDialog::_change_dir(const String &p_new_dir) {
 	} else {
 		String old_dir = dir_access->get_current_dir();
 		dir_access->change_dir(p_new_dir);
-		if (!dir_access->get_current_dir(false).begins_with(root_prefix)) {
+		if (!dir_access->get_current_dir().begins_with(root_prefix)) {
 			dir_access->change_dir(old_dir);
 			return;
 		}