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

Maniphest Tasks: T6958, T7484

Differential Revision: https://secure.phabricator.com/D12574