channels: Add mechanism to patch checkouts of the 'guix channel.

channels: Add mechanism to patch checkouts of the 'guix channel.

* guix/channels.scm (<patch>): New record type.
(apply-patches): New procedure.
(latest-channel-instance)[dot-git?]: New procedure.
Use 'update-cached-checkout' and 'add-to-store' instead of
'latest-repository-commit'.  Call 'apply-patches' when CHANNEL is the
'guix channel.
(%patches): New variable.
* guix/git.scm (url+commit->name): Make public.
* tests/channels.scm ("latest-channel-instances includes channel dependencies")
("latest-channel-instances excludes duplicate channel dependencies"):
Mock 'update-cached-checkout' instead of 'latest-repository-commit'.
Wrap body in 'with-store' and pass the store to 'latest-channel-instances'.