Prompt dir expansion after removing cwd's parent

[Daniel Shahaf <d.s@xxxxxxxxxxxxxxxxxx>]

With 5.9:

     1	% cd /tmp
     2	% PS1='%~%# '
     3	/tmp% mkdir foo foo/bar 
     4	/tmp% cd foo/bar
     5	/tmp/foo/bar% rm -rf /tmp/foo
     6	/tmp/foo/bar% cd ..
     7	/tmp/foo/bar% cd ..
     8	/tmp% 

The prompts on lines 6 and 7 are wrong.

Ideally I'd like to see some indication that cwd has been deleted, or
that a path to cwd couldn't be determined, etc..  Definitely not some
apprently-valid path that points to something else than cwd.

Cheers,

Daniel
(who hasn't forgotten about the widgets shadowing docs thread)