tag: support configuring --sort via .gitconfig

Add support for configuring default sort ordering for git tags. Command
line option will override this configured value, using the exact same
syntax.

Cc: Jeff King <peff@peff.net>
Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>