git-gui: Sort tags descending by tagger date

git-gui: Sort tags descending by tagger date

When trying to create a branch from a tag most people are looking
for a recent tag, not one that is ancient history.  Rather than
sorting tags by their string we now sort them by taggerdate, as
this places the recent tags at the top of the list and the very
old ones at the end.  Tag date works nicely as an approximation
of the actual history order of commits.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>