That way a command like:
guix time-machine --commit=5aeee07cc9 -- describe
goes from 3.4s to 0.5s on a cache hit, even slightly less when passing
the full commit ID.
* guix/inferior.scm (channel-full-commit): New procedure.
(cached-channel-instance): Remove 'instances' top-level variable. Add
'commits' and use it for 'key'. Move 'latest-channel-instances' call to
the cache miss case.