Fixes T4990

Summary: Fix for T4990, using export TERM directly in pre receive hook, tested for git

Test Plan:
pushing into repository over ssh will now not cause remote warning
No entry for terminal type "unknown";
using dumb terminal settings.

Tested with git

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley, Korvin

Projects: #diffusion, #repositories

Maniphest Tasks: T4990

Differential Revision: https://secure.phabricator.com/D9744