Record more details about where a write is taking place while holding a cluster lock

Summary: Ref T4292. This will let the UI and future bin/repository tools give administrators more tools to understand problems when reporting or resolving them.

Test Plan:

• Pushed fully clean repository.
• Pushed previously-pushed repository.
• Forced write to abort, inspected useful information in the database.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T4292

Differential Revision: https://secure.phabricator.com/D15748