_git: Add new arguments introduced in Git v1.8.4
_git: Add status.short and status.branch
_git: Reword --heads to match new meaning
_git: Add two sub-comamnds introduced in v1.8.4

Add _git-check-ignore() and _git-check-mailmap()
_git: Make --batch(-check) accept an argument

Also reword the explanation slightly to make it more compact since more
explanation is needed.