describe: do not use unannotated tag even if exact match

describe: do not use unannotated tag even if exact match

4d23660 (describe: when failing, tell the user about options that
work, 2009-10-28) forgot to update the shortcut path where the code
detected and used a possible exact match.  This means that an
unannotated tag on HEAD would be used by 'git describe'.

Guard this code path against the new circumstances, where unannotated
tags can be present in ->util even if we're not actually planning to
use them.

While there, also add some tests for --all.

Reported by 'yashi' on IRC.

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