Related T703

Closes T255
Ref. D179

- default
- ignoring empty folders
- ignore folder based on pattern in names

Combination of:
- validation on files
- ignore empty folder

Since we compute it anyway, better return it along with the result

Decrease the number of paths to compute to 1 common ancestor (if any):
- Scan only that directory and rehash with new results (data changed)
- Update the resulting objects with those new hashes.
- Update from that directory to the rootdir the existing hashes computation

level for example)