amend: abort early if no username is configured with evolve enabled (issue4211) Amend will reuse the original username if a new one is not provided with -U, but obsolete.createmarkers() only considers ui.username() for the obsolete marker's metadata. Allowing the metadata field to be spoofed seems to defeat the point of the field in the first place. This covers 'evolve amend' and 'ci --amend' with evolve enabled. Without this, the transaction aborts but the parent changeset is set to -1. The corresponding test will be added to evolve separately.