Fixed bug in Highlighter.invalidate() when setting first_invalid_line

The value should not be updated if the current first_invalid_line is less than
the new invalid line index

data/core/doc/highlighter.lua

@@ -46,7 +46,7 @@ end
 
 
 function Highlighter:invalidate(idx)
-  self.first_invalid_line = idx
+  self.first_invalid_line = math.min(self.first_invalid_line, idx)
   self.max_wanted_line = math.min(self.max_wanted_line, #self.doc.lines)
 end