Create index and worktree objects just once

Create the objects for a repository's default index, worktree, and
index+worktree just once. Both for performance (though the gain is
probably negligible), and for future-proofing if we ever add mutable
state to those objects.

And make them properties while we're at it.

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