This fix makes libgit2 capable of parsing annotated tag objects that lack
the optional message/description field.
Previously, libgit2 treated this field as mandatory and raised a tag_error on
such tags. However, the message field is optional.

An example of such a tag is refs/tags/v2.6.16.31-rc1 in Linux:

$ git cat-file tag refs/tags/v2.6.16.31-rc1
object afaa018cefb6af63befef1df7d8febaae904434f
type commit
tag v2.6.16.31-rc1
tagger Adrian Bunk <bunk@stusta.de> 1162716505 +0100
$