Robert Dailey reported confusion on the mailing list about a nested
tag which was most likely created by mistake. Jeff King noted that
this isn't a very common case and creating a tag-to-a-tag can be a
user-error.

Suggest that it may be a mistake with an advice message when
creating such a tag.  Those who do want to create a tag that point
at another tag regularly can turn it off with the usual advice
mechanism.
Reported-by: NRobert Dailey <rcdailey.lists@gmail.com>
Helped-by: NJeff King <peff@peff.net>
Helped-by: NÆvar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: NDenton Liu <liu.denton@gmail.com>
[jc: fixed test style and tweaked the log message]
Signed-off-by: NJunio C Hamano <gitster@pobox.com>