bug#27797: 25.2; text-quoting-style interferes with tree-widget

[Leo Liu]

tree widget in the terminal uses ` in the lower-left corner of a
rectangle, for example

   |
   |
   `--

This is now shown ugly in emacs 25+:

   |
   |
   ‘--

Here is a real example:

[-]Small, fast, modern
 ‘-[-]<101.316.0>
    ‘-[-]<101.317.0>
       ‘-[-]x_sup
          ‘-x_clock

I think this change makes tree-widget look horrible.

Leo