doesn't need to do the same boring replacing I did.
(cherry picked from commit 10c62aa69193a8bc7b46ca24c2ad1d5008489665)

The imported module is called git (as in "import git"), so it's less
confusing to do so than to call everything GitPython.something.

It doesn't use an object's private contents, so let's go...

It is rather intuitive to consider trees as a dict of objects (like
a directory could be seen as a dict of files).

It seems more natural to use a dictionnary for directories, since we
usually want to access them by name, and entry order is not relevant.

Also, finding a particular blob given its name is O(1) instead of O(N).