[zsh] Prevent the current directory from appearing as `~dir` in prompts (#1774)

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

shell/key-bindings.zsh

@@ -56,6 +56,7 @@ fzf-cd-widget() {
     return 0
   fi
   cd "$dir"
+  unset dir # ensure this doesn't end up appearing in prompt expansion
   local ret=$?
   zle fzf-redraw-prompt
   return $ret