Specify HOME when invoking Git commands

Summary: Fixes T2965, see that task for discussion. This is dumb but seems like our best bet.

Test Plan:

• Installed newish version of Git.
• Set HOME on the websever to /var/root (or any other unreadable directory).
• Hit the error described in T2965 when viewing Diffusion.
• Applied this patch.
• Diffusion works.

Reviewers: btrahan, joel

Reviewed By: btrahan

CC: aran, chad

Maniphest Tasks: T2965

Differential Revision: https://secure.phabricator.com/D5994