_git: fix a few "undocumented" options

commit: '--null' is documented
relink: '-h,--help' are never documented for subcommands but always exist
verify-tag: '-v,--verbose' are documented with updated description
diff: '--cumulative' is deprecated -> remove
_git: revert: add '-e,--edit'
_git: Add git-clean -i/--interactive support

 - Add -i and --interactive to _git-clean()
 - Add column.clean configuration variable
_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.