Re: Teach read-tree 2-way merge to ignore intermediate symlinks

> Earlier in 16a4c61, we taught "read-tree -m -u" not to be
> confused when switching from a branch that has a path frotz/filfre
> to another branch that has a symlink frotz that points at xyzzy/
> directory.  The fix was incomplete in that it was still confused
> when coming back (i.e. switching from a branch with frotz -> xyzzy/
> to another branch with frotz/filfre).
> 
> This fix is rather expensive in that for a path that is created
> we would need to see if any of the leading component of that
> path exists as a symbolic link in the filesystem (in which case,
> we know that path itself does not exist, and the fact we already
> decided to check it out tells us that in the index we already
> know that symbolic link is going away as there is no D/F
> conflict).
> 
> Signed-off-by: Junio C Hamano <gitster@pobox.com>