R1:f424f9f2d206

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.