Fix to use $input_id to repopulate data

This was using `$this->Tag->id()` instead of `$this->Tag->input_id()` to read in the data. This happened to work pre-Repeaters, but wasn't designed to be used that way.

Switching to use `$this->Tag->input_id()` should work correctly both pre- and post- Perch 2.3 and should even work within Repeaters.