describe: --match=<pattern> must limit the refs even when used with --all

The logic to limit the refs used for describing with a matching pattern
with --match=<pattern> parameter was implemented incorrectly when --all
is in effect.  It just demoted a ref that did not match the pattern to
lower priority---if there aren't other refs with higher priority
that describe the given commit, such an unmatching ref was still used.

When --match is used, reject refs that do not match the given
criteria, so that with or without --all, the output will only use
refs that match the pattern.

Signed-off-by: Junio C Hamano <gitster@pobox.com>
Merge branch 'jc/describe' into pu

* jc/describe:
  describe: --match=<pattern> must limit the refs even when used with --all