Test for packed tags in git-describe output

Test for packed tags in git-describe output

In c374b91c ("git-describe: use tags found in packed-refs correctly")
Junio fixed an issue where git-describe did not parse a tag object it
obtained from a packed-refs file, as the peel information was read in
from packed-refs and not the tag object itself.

This new test case verifies the fix listed above is functioning, and
does not have a regression in the future.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>