plumbing: object, Fix tag message decoding
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 <33770123f9ea2d53c98d72f7d006ef0f2610ab9f@gmail.com>

go-git/go-git,mcuadros/go-git,mcuadros/go-git,src-d/go-git,go-git/go-git,src-d/go-git