'gettext -- "--cached cannot be used with --files"' does not work

Hi,

I just noticed that

	'gettext -- "--cached cannot be used with --files"' 

in git-submodule.sh generates

	#: git-submodule.sh:588
	msgid "--"
	msgstr ""

in *.po files. You can this e.g. in po/de.po.

This is not what commit message says :)

    gettextize: git-submodule "cached cannot be used" message
    
    Gettextize the "--cached cannot be used with --files" message. Since
    this message starts with "--" we have to pass "--" as the first
    argument. This works with both GNU gettext 0.18.1 (as expected), and
    the gettext(1) on Solaris 10.

$ gettext --version gettext (GNU gettext-runtime) 0.18.1