Removed support for git versions older than 2.5.

Removed support for git versions older than 2.5.

This entirely removes Git.BuildVersion, which avoids the possibility that
git-annex will behave differently based on the version of git it was built
with, rather than the version it's used with.

Debian oldoldstable is the oldest version of git that git-annex needs to
support, since it's used in the amd64ancient build.

cabal configure will fail if the git version is too old.

Sponsored-by: Nicholas Golder-Manning