Refactor and document `Git`.

Library/Homebrew/dependency_collector.rb

Library/Homebrew/dev-cmd/audit.rb

Library/Homebrew/dev-cmd/bottle.rb

Library/Homebrew/dev-cmd/extract.rb

Library/Homebrew/dev-cmd/pull.rb

Library/Homebrew/diagnostic.rb

Library/Homebrew/extend/git_repository.rb

Library/Homebrew/formula.rb

Library/Homebrew/formula_installer.rb

Library/Homebrew/system_config.rb

Library/Homebrew/tap.rb

Library/Homebrew/test/tap_spec.rb

Library/Homebrew/test/utils/git_spec.rb

Library/Homebrew/utils/git.rb