When destorying a repository, print a notification about removing the working copy

Summary:
Fixes T12946. bin/remove destroy does not remove working copies: it's more dangerous than usual, and we can't do it in the general (clustered) case.

Print a notification message after destroying a repository.

Test Plan:

• Destroyed a repository, got a hint about the working copy.
• Destroyed a task, things worked normally.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12946

Differential Revision: https://secure.phabricator.com/D18313