Pressing Cancel while editing a reusable block does not reset the block preview

[noisysocks]

Issue Overview

Pressing Cancel while editing a reusable block should discard any unsaved changes that have been made to that block.

Steps to Reproduce (for bugs)

1. Insert a reusable block
2. Select it and click Edit
3. Make some changes to the block
4. Click Cancel

Expected Behavior

The block should return to how it looked before Edit was clicked.

Current Behavior

The block retains the changes made in step 3, which does not reflect how the post will look once published or previewed.

Possible Solution

The changes to the referenced block's attributes should be kept in local state until Save is clicked, i.e. revert this recent change.

Screenshots / Video

Related Issues and/or PRs

This regression was introduced by #5228.