Build Git manually

Summary:
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.

Fixes T682.

Test Plan: git --version, ensure completion works

Reviewers: dereckson

Reviewed By: dereckson

Subscribers: Nasqueron Docker deployment squad

Tags: Docker images

Maniphest Tasks: T682

Differential Revision: https://devcentral.nasqueron.org/D2368