Dont overwrite existing contributors during the contributor update task on release

[jacobheun]

If you manually add contributors to the respective field in package.json, if those contributors don't show up as an Author in the git logs, they are removed because aegir does a full replace based on the git logs. This is problematic for Co-Authors, because git log doesn't register them as authors.

Ideally the contributors task would pick up Co-Authors in the commit logs, but that is not easy to do at the moment via the git cli. In the interim, merging the existing list and the list from the logs would be ideal.

Ref: multiformats/js-multistream-select#54 (comment)