Build Git manually
Both Arcanist and the shell this image provide need a rather
up-to-date Git version to work correctly.
This change offers to compile Git manually, which is rather straightforward.
That allows to keep Git in sync with releases, not with Debian versions.
Test Plan: git --version, ensure completion works
Reviewed By: dereckson
Subscribers: Nasqueron Docker deployment squad
Tags: Docker images
Maniphest Tasks: T682
Differential Revision: https://devcentral.nasqueron.org/D2368