PS-embedded comsub `\w' evaluation doesn't replace $HOME with ~

[Grisha Levit]

For example:

$ PS1='\w $(echo \w) $ '
~ /home/levit $

Obviously, this is easy enough to work around in most cases by
replacing \w with ${PWD/#$HOME/~}.  I had a silly application for
calculating prompt length and this was throwing it off, but this
behavior has been there since at least 2.05b so not sure if it's worth
changing..