plumbing: object, Fix tag message decoding

The Decode method was adding one too many newlines to the tag message,
causing signature verification to fail. This is because in signed tags
produced by `git`, there is a newline after the PGP signature block,
resulting in `messageAndSig` having one extra (empty) `[]byte` element.
This caused `t.Message` to receive one extra newline.

Signed-off-by: Spencer Judd <spencercjudd@gmail.com>

Author: scjudd

plumbing/object/tag.go

@@ -141,7 +141,7 @@ func (t *Tag) Decode(o plumbing.EncodedObject) (err error) {
 			if pgpsig {
 				if bytes.Contains(l, []byte(endpgp)) {
 					t.PGPSignature += endpgp + "\n"
-					pgpsig = false
+					break
 				} else {
 					t.PGPSignature += string(l) + "\n"
 				}