Emacs mode: automatically cd up to root of worktree

git's emacs mode automatically finds the root of the worktree, so that
the user doesn't have to. Teach StGit's emacs mode the same trick by
borrowing the git-get-top-dir function from git.

The borrowed code was written by Alexandre Julliard <julliard@winehq.org>.

Signed-off-by: Karl Hasselström <kha@treskal.com>