FIX: Fixes #2389
- Prevent circular references in `GridFieldAddExistingAutocompleter` when linking DataObjects whose ID matches the current object to which the gridfield is attached.
FIX: Modified fix for #2389 to ensure existing tests pass.
Merge pull request #2390 from phptek/2389

Prevent circular refs in `GridFieldAddExistingAutocompleter` when linking DataObjects whose ID == current object's ID