Replace subversion with git in the installation DVD

I assume the original reason for Subversion's inclusion was to checkout
the base system. But now we use git for that.

Reviewed by: gjb
Sponsored by: Axcient
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D35329

(cherry picked from commit 55c8093f733242cd72341ba1126637047cf16ca3)