Fixes #1549

This allows the completion script to be sourced more than once while
still behaving correctly. Previously, the script would attempt to
replace `__git_list_all_commands` blindly, which would yield
multiply-replaced calls like
`__git_list_all_commands_without_hub_without_hub`.

Also removed instructions and references to set up git auto-completion.

Signed-off-by: NDavid Calavera <david.calavera@gmail.com>

Previus version was written for bash v4 and was causing syntax error
when used with older versions of bash that still ship with most systems.

References 3565094d
Closes #359

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