Another keep-prefix problem

[Wayne Davison <wayned@users.sourceforge.net>]

I noticed one other problem with the current zsh (in CVS) and how it
handles keeping the prefix.  After Bart's last set of tweaks, this
works:

   % ls $var1/foo/$var2/bar<TAB>

But leaving off the leading variable results in a useless expansion:

   % ls /full/path/foo/$var/bar<TAB>

When the filename only has an embedded variable, it is expanded.
Here's a (completely usless) failure case:

   zsh -f
   % autoload -U compinit
   % compinit
   % zstyle ':completion:*' completer _expand _complete
   % zstyle ':completion:*:expand:*' keep-prefix true
   % bindkey '\t' complete-word
   % www=/var/www/html
   % mkdir $www/subdir
   % echo ~
   /home/wayne
   % ls -l ~/../..$www/sub<TAB>

... properly expands to 'subdir' ...

   % ls -l /home/wayne/../..$www/sub<TAB>

... does not.

..wayne..