amend: do a bare kill of temporary changeset Before this changeset the temporary changeset created by amend was made a precursors on the amend result. This add unnecessary complexity to the obsolescence graph. This temporary commit will probably disappear in the future. It is an unwanted byproduct of amend that nobody cares about.

amend: add noise in extra to avoid creating obsolescence cycle (issue3664) Obsolescence cycle are bad and should be avoided as much as possible. The current amend implemented touch changeset meta data as few as possible. This make is easy for amend to result in the same node than a precursors. We add some deterministic noise in extra to avoid this. In practice, the hex of the amended changeset is stored in 'amend_source' extra key.

changelog: extract description cleaning logic in a dedicated function The amend logic have use for it.