Add author information to commit message when squashed on merge

When using the "Squash on merge" on merge option the authors of the commit are lost, except the user that opened the merge request. When there are multiple contributors this is misleading, particularly when reading the commit log locally.

Proposal

If there were multiple authors or the commit authors differ from the user who opened the merge request, the commit message for the squashed commit should list them using the Co-Authored-By notation (https://git.wiki.kernel.org/index.php/CommitMessageConventions). Each author should be listed once in commit order.

For example, https://gitlab.com/gitlab-org/gitlab-ee/commit/0d812f1bc783e59ecc4ad0dad01438dc5175c2de (MR https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/3524/commits) should have the message:

Add hashed storage warning to upgrade guide

Co-Authored-By: James Ramsay <jramsay@gitlab.com>
Co-Authored-By: Ernst van Nierop <ernst@gitlab.com>