the signature block; existing content will be overwritten
signed data; this is the commit contents minus the signature block; existing content will be overwritten
the repository in which the commit exists
the commit from which to extract the data
the name of the header field containing the signature block; pass null to extract the default 'gpgsig'
0 on success, git_error_code.GIT_ENOTFOUND if the id is not for a commit or the commit does not have a signature.
Extract the signature from a commit
If the id is not for a commit, the error class will be git_error_t.GIT_ERROR_INVALID. If the commit does not have a signature, the error class will be git_error_t.GIT_ERROR_OBJECT.