Make \x the same as \X

Up to now, in normal locales \x was essentially the same as \X, except
that it errored if given a value > 0x7f.

That's kind of annoying and useless.

A subtle change is that `\xHH` now represents the character (if any)
encoded by the byte value "HH", so even for values <= 0x7f if that's
not the same as the ASCII value we would diverge.

I do not believe anyone has ever run fish on a system where that
distinction matters. It isn't a thing for UTF-8, it isn't a thing for
ASCII, it isn't a thing for UTF-16, it isn't a thing for any extended
ASCII scheme - ISO8859-X, it isn't a thing for SHIFT-JIS.

I am reasonably certain we are making that same assumption in other
places.

Fixes #1352

src/highlight.cpp

@@ -604,10 +604,7 @@ static void color_string_internal(const wcstring &buffstr, highlight_spec_t base
                                 in_pos++;
                                 break;
                             }
-                            case L'x': {
-                                in_pos++;
-                                break;
-                            }
+                            case L'x':
                             case L'X': {
                                 max_val = BYTE_MAX;
                                 in_pos++;