(cherry picked from commit 657444d75d7b2d6913c2d491841b9bd70c8bd289)

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 is better to have an explicit list of variables for the constructors,
be it only to avoid mispelled arguments.

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).

This is a simplification of the tree baking code.

As a matter of consequency, Tree.construct() and
tree.construct_initialize() have been killed, and repo.tree() has lost
the "paths" argument. This is not a problem since one can just have the
same result with:

     dict(k, o for k, o in tree.items() if k in paths)

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).