Also don't match the root directory (#216)

besides the `$HOME` directory.

A `z /` call will cd to the root directory anyway, because of how the
code handles completion:

    # if we hit enter on a completion just go there
    case "$last" in
        # completions will always start with /
        /*) [ -z "$list" -a -d "$last" ] && builtin cd "$last" && return;;
    esac

z.sh

@@ -56,8 +56,8 @@ _z() {
     if [ "$1" = "--add" ]; then
         shift
 
-        # $HOME isn't worth matching
-        [ "$*" = "$HOME" ] && return
+        # $HOME and / aren't worth matching
+        [ "$*" = "$HOME" -o "$*" = '/' ] && return
 
         # don't track excluded directory trees
         local exclude