submodule deinit: require '--all' instead of '.' for all submodules

The discussion in [1] pointed out that '.' is a faulty suggestion as
there is a corner case where it fails:

> "submodule deinit ." may have "worked" in the sense that you would
> have at least one path in your tree and avoided this "nothing
> matches" most of the time.  It would have still failed with the
> exactly same error if run in an empty repository, i.e.
>
>        $ E=/var/tmp/x/empty && rm -fr "$E" && mkdir -p "$E" && cd "$E"
>        $ git init
>        $ rungit v2.6.6 submodule deinit .
>        error: pathspec '.' did not match any file(s) known to git.
>        Did you forget to 'git add'?
>        $ >file && git add file
>        $ rungit v2.6.6 submodule deinit .
>        $ echo $?
>        0

So instead of a pathspec add the '--all' option to deinit all submodules
and add a test to check for the corner case of an empty repository.

The code only needs to learn about the '--all' option and doesn't
require further changes as `git submodule--helper list "$@"` will list
all submodules when "$@" is empty.

[1] http://news.gmane.org/gmane.comp.version-control.git/289535

Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Stefan Beller <sbeller@google.com>
Reviewed-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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