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).

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)

Move timestamp normalization to another function to make it more easily
movable.

This function wasn't in sync with what's supposed to be our revision
schema

so no bytes, and we enforce during the checksum computation function

T270