Create a draft patch set when a draft patch set is rebased

Currently rebasing a draft patch set will create a non-draft patch set
which is published. It is unexpected that rebasing a draft patch set
publishes the modifications done in the draft patch set.

Bug: issue 1558
Change-Id: Iaf3b2d5b34076fbd85958fffd4bb1369bc8fd6db
Signed-off-by: Edwin Kempin <edwin.kempin@sap.com>