tag: advise on nested tags

tag: advise on nested tags

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: Robert Dailey <redacted>
Helped-by: Jeff King <redacted>
Helped-by: Ævar Arnfjörð Bjarmason <redacted>
Signed-off-by: Denton Liu <redacted>
[jc: fixed test style and tweaked the log message]
Signed-off-by: Junio C Hamano <redacted>