Write a trivial log when starting a repository operation

Summary:
Depends on D19672. Ref T13197. See PHI873. This writes a trivial log when we begin acting on a working copy and makes it look reasonable in the UI.

This is mostly just to prove that logging works properly.

Test Plan:

Reviewers: amckinley

Reviewed By: amckinley

Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam

Maniphest Tasks: T13197

Differential Revision: https://secure.phabricator.com/D19673