Do not add selection with newlines in replace

If the selected text containes newlines it doesn't make sense to
use it as the initial text in the "replace text" command view.

Do not use the selected text if a newline is found in the selection.

Fix #511.

data/core/commands/findreplace.lua

@@ -135,7 +135,9 @@ command.add("core.docview", {
   end,
 
   ["find-replace:replace"] = function()
-    replace("Text", doc():get_text(doc():get_selection(true)), function(text, old, new)
+    local selected_text = doc():get_text(doc():get_selection())
+    local has_newlines = selected_text:find("\n", 1, true)
+    replace("Text", has_newlines and "" or selected_text, function(text, old, new)
       if not find_regex then
         return text:gsub(old:gsub("%W", "%%%1"), new:gsub("%%", "%%%%"), nil)
       end