channels: Make 'validate-pull' call right after clone/pull.

This should come before patching, authentication, etc.

* guix/channels.scm (latest-channel-instance): Add #:validate-pull
parameter and honor it.  Return a single value: the instance.
(ensure-forward-channel-update): Change 'instance' parameter to 'commit'
and adjust accordingly.
(latest-channel-instances): Adjust to 'latest-channel-instance' changes.
* guix/scripts/pull.scm (warn-about-backward-updates): Change 'instance'
parameter to 'commit' and adjust accordingly.
* tests/channels.scm ("latest-channel-instances #:validate-pull"):
Likewise.