R1:fad75f939de5

Improve messaging around repository locks

Summary:
Fixes T6958. Ref T7484.

- When we collide on a lock in `bin/repository update`, explain what that means.
- GlobalLock currently uses a "lock name" which is different from the lock's actual name. Don't do this. There's a small chance this fixes T7484, but I don't have high hopes.

Test Plan: Ran `bin/repository update X` in two windows really fast, got the new message in one of them.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley…