<bugfix>removed self reference in static method in persistence package</bugfix>
<bugfix>removed self reference in static method in persistence package</bugfix>

packages/persistence/init.py:
- PersistentPath.git_compute_hash: change self to None
- PersistentPath.git_compute_file_hash: change self to None
- PersistentPath.compute: catch exception for git_compute_hash call
                          and add module to exception

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