Don't rely on internals from the git completion. Instead set up the
environment and then simply call _git and let it do the completion as
it see fit.
See #292.
It's only supported as a command option and the command will fail if
-d is used as a global option. Instead of completing it in the wrong
place, don't complete it at all as it can be considered an "internal"
switch (it's not mentioned in the manual).
