R1:c8529787f345

Provide TERM=dumb for Mercurial commit hooks

Summary: Fixes T7119.

Test Plan: Will make someone test.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: cspeckmim, epriestley

Maniphest Tasks: T7119

Differential Revision: https://secure.phabricator.com/D12182