Forward the username when updating the pull-request
This way the update of the PR is linked to the person who made the push
which solves the bug we had where when a PR was rebased it was shown as
having been rebased by the author of the PR rather than the person who
actually rebased the PR.
Fixes https://pagure.io/pagure/issue/4569
Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>