Previously the 'commit' field would always be interpreted as a commit
ID. This change adds flexibility, allowing for things like:
guix time-machine --commit=v1.2.0 -- describe
* guix/channels.scm (channel-reference): Use 'tag-or-commit' rather than 'commit'.
* guix/inferior.scm (channel-full-commit): Likewise.
* doc/guix.texi (Invoking guix pull): Document it.
(Invoking guix time-machine): Likewise.
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