This updates our emacs-magit package to the latest version, which
has the following benefits:

1) The test suite is now enabled.
2) The libgit2 C library is now accelerating some of the Magit tasks
3) Using the emacs-build-system means the Magit autoload file is
now byte compiled.
4) No custom patching is done.

The other updates were required to prevent breaking some of the packages
which had a dependency on emacs-magit.

Maxim