<bugfix>Fix issue with intermediate persistent files not being found in the store when a uuid is already assigned.</bugfix>
<bugfix>Fix issue with intermediate persistent files not being found in the store when a uuid is already assigned.</bugfix>

As Ross Harder found, new intermediate persistent files are created when a reference exists for that file even if the file does not exist in the store.  We now check this and create a new reference only when that reference already exists.

vistrails/packages/persistence/init.py:
  - PersistentPath.updateUpstream: check if the reference actually exists and if it doesn't, check the signatures to see if an existing file can be reused.

celiafish/VisTrails,minesense/VisTrails,minesense/VisTrails,celiafish/VisTrails,celiafish/VisTrails,minesense/VisTrails,hjanime/VisTrails,Nikea/VisTrails,VisTrails/VisTrails,VisTrails/VisTrails,VisTrails/VisTrails,Nikea/VisTrails,Nikea/VisTrails,Nikea/VisTrails,VisTrails/VisTrails,hjanime/VisTrails,hjanime/VisTrails,VisTrails/VisTrails,celiafish/VisTrails,hjanime/VisTrails,minesense/VisTrails,hjanime/VisTrails,minesense/VisTrails