Fix MR creation on fork of deleted project
selected_target_project helper method is used for actions where
target_project_id is a top-level parameter, not nested under the
Even though that logic was wrong, if the target project wasn't a fork,
or was a fork of an active project, this would work through
coincidence (although it might check the wrong value for
If a project was a fork of a deleted project, however, then
selected_target_project would be
nil, causing a 500 error. Ensure
that there is always a target project to compare to.
Closes #732 (closed).