This `_hub` completion script should correctly wrap modern versions of
the `_git` scripts distributed by both zsh (native) and git
(bash-based).  The bash-based version could use a bit more love to
complete arguments to the actual subcommands.  Right now it can only
complete the names of the subcommands themselves.

The `compdef` that was previously emitted by `hub alias -s` prevents the
new `_git` script from working, so it has been eliminated.

Fixes #231