channels: Allow news entries to refer to a tag.

channels: Allow news entries to refer to a tag.

Suggested by Ricardo Wurmus <rekado@elephly.net>.

* guix/channels.scm (<channel-news-entry>)[tag]: New field.
(sexp->channel-news-entry): Accept either 'commit' or 'tag' in 'entry'
forms.
(resolve-channel-news-entry-tag): New procedure.
(channel-news-for-commit): Move 'with-repository' form one level
higher.  Call 'resolve-channel-news-entry-tag' on all the news entries.
* guix/tests/git.scm (populate-git-repository): Add clause for 'tag'.
* tests/channels.scm ("channel-news, one entry"): Create a tag and add
an entry with a tag.  Check that the tag is resolved and also visible in
the <channel-news-entry> record.
* doc/guix.texi (Channels): Mention tags in news entries.