[PATCH Cogito] Fix 'cg-*-id' to make 'make-cg-asciidoc' happy

Hi !

When bumping the Gentoo package to cogito-0.15 I came accross a
problem while making the documentation. cg-tree-id, cg-parent-id and
cg-commit-id didn't have an USAGE variable because they don't use it;
however this breaks the documentation generation.

This trivial patch fixes it.

Cheers,
Ferdy

-- 
Fernando J. Pereda Garcimartín
Gentoo Developer (Alpha,net-mail)
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