Blender Git Loki

Stretch To: fix initialization of the constraint with copy-on-write.

The constraint is supposed to automatically initialize the rest
length when it is first evaluated, so now that evaluation is done
on a separate copy it also has to copy the value to the master
instance - or newly created constraints will be broken.

Since this is supposed to happen once at constraint creation,
implementing as a search for now instead of adding new fields.