Extract the signature from a commit
If the id is not for a commit, the error class will be
GITERR_INVALID. If the commit does not have a signature, the
error class will be GITERR_OBJECT.
the name of the header field containing the signature block; pass NULL to extract the default 'gpgsig'
0 on success, GIT_ENOTFOUND if the id is not for a commit or the commit does not have a signature.