Joining cg-*-id

Hello!

I believe Cogito should keep the command list short.  When the *-id
utilities got the cg-prefix, the list became longer by 3 commands.

What if we join cg-commit-id, cg-parent-id and cg-tree-id into one cg-id
(or cg-admin-id) utility?  It would work like cg-commit-id in absence of
switches, -p would make it work like cg-parent-id, and -t would make it
cg-tree-id.

Alternatively, this functionality should go to git.  It's low-level
enough to be there.

-- 
Regards,
Pavel Roskin