Translate Chapter2 "Git Basics"

1. viewing-history.asc: git log --before/afterの「含む/含まない」問題

  修正された原文に合わせて「含まない」ニュアンスに統一

1. recording-changes.asc: The “CONTRIBUTING.md” file appears under a section named “Changed but not staged for commit”

  Changes not staged for commit: が正

1. recording-changes.asc: it shows up under the ``Changed but not updated'' (that is, _unstaged_)

  Changes not staged for commit: が正？