Migrating freebsd-update-build to git

[Alan Somers]

Migrating ports to git was a big milestone, but there's at least one other
repository that needs migrating.
https://svnweb.freebsd.org/base/user/cperciva/freebsd-update-build/ is used
to build official releases, and it's also used by people who have private
freebsd-update servers.  Or I should say, it _was_ used.  It's orphaned
code now.  Could you please migrate it to git, too?
-Alan