Set uploader to current user in "patchset-created" event upon rebasing

When rebasing a change through the Web-UI (both old and new
ChangeScreen), the corresponding patchset-created event had its
patchSet.uploader set to the Change's owner instead of the current
user. We now set it to the current user, so stream-events consumers
can properly detect who uploaded the rebased patch set.

Upstream-Bug: https://code.google.com/p/gerrit/issues/detail?id=2493
Bug: 60781
Change-Id: Iea9459c7015cde9e869dad5946bd1d3b2db5d51a