git: Run subprocess.check_output with universal_newlines=True.

In this way, we obtain text (str) output with '\n' as line ends.
git: Add a new property to Repo: name.
git: Add a new property to Branch: commit.
git: Fix Repo.get_commit_for_branch().
git: Change the name and signature of Repo._get_commit_from_git_show_with_args().