Tweak --limit for bin/phd log

Summary:
Ref T5405.

  - `--limit` wasn't actually used anywhere.
  - Make it mean "the N newest lines".

Test Plan: Ran `bin/phd log`, `bin/phd log --limit 3`.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T5405

Differential Revision: https://secure.phabricator.com/D10385