git: 'update-cached-checkout' returns the commit relation.

* guix/git.scm (update-cached-checkout): Add #:starting-commit
parameter.  Call 'commit-relation' when #:starting-commit is true.
Always return the relation or #f as the third value.
(latest-repository-commit): Adjust accordingly.
* guix/import/opam.scm (get-opam-repository): Likewise.
* tests/channels.scm ("latest-channel-instances includes channel dependencies")
("latest-channel-instances excludes duplicate channel dependencies"):
Update mock of 'update-cached-checkout' accordingly.