describe: when failing, tell the user about options that work

describe: when failing, tell the user about options that work

Users seem to call git-describe without reading the manpage, and then
wonder why it doesn't work with unannotated tags by default.

Make a minimal effort towards seeing if there would have been
unannotated tags, and tell the user.  Specifically, we say that --tags
could work if we found any unannotated tags.  If not, we say that
--always would have given results.

Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>