Enforce convention on directory name without trailing /.
At the moment, the `git.walk_and_compute_sha1_from_directory` injected
the rootdir with a possible trailing / (input from client).

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)