vfs: change /save/latest back to a symlink to the latest save

vfs: change /save/latest back to a symlink to the latest save

The current, reworked vfs presents /somesave/latest as if it actually
is the latest commit.  Change it back to a symlink to the latest save
to roughly match the previous behavior -- though now it's a link to
the save name, not to the (removed) /.commit/ subtree.

To restore the link, reintroduce the concept of a fake
symlink (i.e. one that has no corresponding blob in the repository).

Signed-off-by: Rob Browning <rlb@defaultvalue.org>
Tested-by: Rob Browning <rlb@defaultvalue.org>