Move Gitlab::Git out of gitlab core

Gemfile

Gemfile.lock

config/initializers/5_backend.rb

lib/gitlab/git/blame.rb

lib/gitlab/git/blob.rb

lib/gitlab/git/commit.rb

lib/gitlab/git/compare.rb

lib/gitlab/git/diff.rb

lib/gitlab/git/repository.rb

lib/gitlab/git/stats.rb

lib/gitlab/git/tree.rb

spec/lib/gitlab/git/commit_spec.rb

spec/lib/gitlab/git/diff_spec.rb

spec/lib/gitlab/git/repository_spec.rb

spec/lib/gitlab/git/stats_spec.rb

spec/support/test_env.rb