Use Gitlab::Git:: for git features across application

app/controllers/blame_controller.rb

app/controllers/compare_controller.rb

app/models/commit.rb

app/models/gollum_wiki.rb

app/models/repository.rb

app/models/wiki_page.rb

app/views/blame/show.html.haml

app/views/compare/show.html.haml

app/views/wikis/show.html.haml

lib/gitlab/git/commit.rb

spec/support/test_env.rb