The zsh version of the cd widget sets the variable `dir` to the path of
the target directory before invoking `cd`. This causes zsh to treat the
target directory as a named directory, which has the effect of zsh
substituting '%~' with '~dir' instead of the proper path when it
performs prompt expansion.
This commit will cause the widget to unset `dir` before redrawing the
prompt to fix this issue.
Details of zsh prompt expansion can be found in:
http://zsh.sourceforge.net/Doc/Release/Prompt-Expansion.html
