Save/Revert are not disabled for snap with categories

[bartaz]

Found when testing #1772

Steps to reproduce the problem

1. Go to listing page of a snap with category set
2. Change anything on listing page (add something to title)
3. Save/Revert button should get enabled
4. Without saving anything remove the change done before (change the title back)
5. Save/Revert button should get disabled (there are no changes to save), but doesn't

Expected behaviour

Save/Revert button should get properly disabled if changes are removed.

Debugging

It seems that when snap has categories they always show up in the diff that we use to check if anything changed.