Set TERM to prevent No entry for terminal type "unknown" messages during fetch

Summary:
Fetches cause output in /var/tmp/phd/log/daemons.log as
follows:

PHLOG: 'Unexpected output while updating repository "rREPONAME": No entry for terminal type "unknown";
using dumb terminal settings.
' at [/path/to/phabricator/src/applications/repository/daemon/PhabricatorRepositoryPullLocalDaemon.php:455]

These warnings come from PHP itself. Silence these warnings by providing a
known value for TERM before shelling out to the PHP script.

See also D9744 (reverted in D11644) and T4990/T7119, which are a similar issue,
but in the pre-receive hooks, not the pull daemons.

Test Plan:
Enabled in production, observed errors to be silenced and
no SSH hangs

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D17100