_git: reword _git-config TODO

Support for merge drivers, filter drivers, and diff drivers is
missing.
_git: support completing remote branches without <remote>/ prefix

this is used in git 1.8's
git checkout <branch>
as a shorthand for
git checkout -b <branch> --track <remote>/<branch>
in case <branch> exists on exactly one remote and is not a local branch
_git: fix some typos