GNU Emacs modes for Git-related files.

* git-commit-mode - A major mode for editing Git commit messages
  according to the guidelines by Tim Pope.
* git-rebase-mode - A major mode for git-rebase-todo files created by
  git rebase -i
* gitconfig-mode - A 'conf-mode'-derived major mode for editing
  .gitconfig files.
* gitignore-mode - A 'conf-mode'-derived major mode for editing
  .gitignore files.
* gitattributes-mode - A major mode for editing .gitattributes files.
