Fix git-verify-tag for light-weight tags

Fix git-verify-tag for light-weight tags

It currently exits printing "git-cat-file SHA1: bad file", while
instead we must just abort the verification for light-weight
tags (e.g. referring to commit objects).

[jc: tag objects can tag anything not just commits, so I fixed
 up the original patch slightly.  you should be able to validate
 a signed tag that points at a blob object. ]

Signed-off-by: Paolo 'Blaisorblade' Giarrusso <blaisorblade@yahoo.it>
Signed-off-by: Junio C Hamano <junkio@cox.net>