Pretty-print tagger dates.

Pretty-print tagger dates.

We can show commit objects with human readable dates using
various --pretty options, but there was no way to do so with
tags.  This introduces two such ways:

$ git-cat-file -p v1.2.3

shows the tag object with tagger dates in human readable format.

$ git-verify-tag --verbose v1.2.3

uses it to show the contents of the tag object as well as doing
GPG verification.

Signed-off-by: Junio C Hamano <junkio@cox.net>