While playing with cg-ls, I tried:

% cg-ls
... snip ...
100644  blob    bc607fd55f6ce4e56ce87766369b5d4d55ec79af        object.h
100755  blob    f35877a6aa5b68d2fb4a388dcfa9b3e64262604e        parent-id
040000  tree    bfb75011c32589b282dd9c86621dadb0f0bb3866        ppc
100644  blob    d922305ee0f5583bdfcb629f6d4061e11e0fa859        read-cache.c
100644  blob    1ad7ffc555b635fe57fa7834b12d71ff576be065        read-tree.c
... snip ...
% cg-ls bfb75011c32589b282dd9c86621dadb0f0bb3866       <-- the ppc tree ID
Invalid id: bfb75011c32589b282dd9c86621dadb0f0bb3866
usage: cat-file [-t | tagname] <sha1>
usage: cat-file [-t | tagname] <sha1>
Invalid id:


Shouldn't cg-ls give a listing of a sub-tree?  The cg-help says it takes
a TREE-ID?

The problem seems to be that tree-id really only accepts a commit-id and
returns the TREE-ID of that commit.

So I modified commit-id, tree-id and parent-id to make them more similar
in coding style, force "short-id" names to be at least 4 lower case
letters, and have tree-id accept short, unambiguous ID's and bare SHA1-ID's.

Patch attached.