Correct priority of lightweight tags in git-describe.

Correct priority of lightweight tags in git-describe.

We really want to always favor an annotated tag over a lightweight
tag when describing a commit.  Unfortunately git-describe wasn't
doing this as it was favoring the depth attribute of a possible_tag
over the priority.  Now priority is the highest sort and we only
consider a lightweight tag if no annotated tags were identified.

Rather than searching for the minimum tag using a simple loop we
now sort them using a stable sort algorithm, this way the possible
tags display in order if --debug gets used.  The stable sort helps
to preseve the inherit topology/date order that we obtain during
our search loop.

This fix allows the tests in t6120-describe.sh to pass.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>