fix(path): Windows parent root directory (#504)

Author: jamestrew

lua/plenary/path.lua

@@ -696,7 +696,11 @@ end
 local _get_parent = (function()
   local formatted = string.format("^(.+)%s[^%s]+", path.sep, path.sep)
   return function(abs_path)
-    return abs_path:match(formatted)
+    local parent = abs_path:match(formatted)
+    if parent ~= nil and not parent:find(path.sep) then
+      return parent .. path.sep
+    end
+    return parent
   end
 end)()