Fix T685 out of spite.

While we wait for a cleaner refactoring of this code, this fixes the immediate
clogging of the database with bogus data issue.

Closes T255
Ref. D179

This permits to reuse the same logic for different clients (loader-dir,
loader-tar, loader-svn)

(Tests were lost)

This actually is supposed to replace
walk_and_compute_sha1_from_directory.
The data structure used here is better at handling updates.

(Code that actually got lost and rewritten - Tests are definitely lost though)

Keep the old api (since i don't measure the impacts on other modules yet).

+ Improve docstring

(Code that actually got lost and rewritten)

In some corner case, the changed paths can reference a previous ignored
folder (thus not existing in the data structure)

The empty folder was not previously in the objects structure.
So we need to add it as child of its parent for the update.

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)