Cancel Tree.refresh when new root set

[colin-grant-work]

What it does

On application startup, these errors are logged by the scm package:

This PR addresses the error by modifying the logic for handling set root on the TreeImpl so that it cancels a refresh cycle when a new root is set. Previously, refresh would continue running and eventually discover that the root with which it began no longer corresponded to the current root of the tree, producing the error.

How to test

1. Start the application.
2. Assert that the errors noted above no longer occur.
3. Trees should continue to work normally: since refresh is called every time set root runs, the final refresh should still run to completion.

Review checklist

• As an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• As a reviewer, I agree to behave in accordance with the review guidelines

[msujew]

Looks good to me 👍 I can confirm that the issue exists on master and is addressed by this change.

[vince-fugnitto]

LGTM 👍