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>

git-verify-tag.sh

@@ -1,8 +1,12 @@
 #!/bin/sh
 . git-sh-setup || die "Not a git archive"
 
-tag=$(git-rev-parse $1) || exit 1
+type="$(git-cat-file -t "$1" 2>/dev/null)" ||
+	die "$1: no such object."
 
-git-cat-file tag $tag > .tmp-vtag || exit 1
+test "$type" = tag ||
+	die "$1: cannot verify a non-tag object of type $type."
+
+git-cat-file tag "$1" > .tmp-vtag || exit 1
 cat .tmp-vtag | sed '/-----BEGIN PGP/Q' | gpg --verify .tmp-vtag - || exit 1
 rm -f .tmp-vtag