_git: make git submodule <cmd> completion work

The -A parameter in the main completion stopped completion
in the subcommands from working. Remove it.
_git: correctly use __git_guard_number in some cases

The space is relevant to make the help text work as intended.
_git: completion updates to match latest git v1.8.3
_git: completion updates to match latest git v1.8.3 part 2
_git: add new configuration options to match latest git v1.8.3
_git: reword _git-config TODO to reflect missing merge/diff/filter drivers
_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