Add the Debusine login to .changes files signed by Debusine

On the ELTS side, Freexian presently wants contributors to sign their uploads using debusine provide-signature to establish traceability of who performed the upload. It would be nice to be able to combine automatic acceptance in the absence of failure with traceability. This is possible if Debusine textually includes the login of the requesting user in a custom field of the .changes file to be signed. Colin notes that for this to be sensible, Debusine would have to always strip this field from .changes files provided by users before signing them.