linting: switch the mark off when the linter starts

Because the highlighting hinders the display of affected lines,
and, more importantly, only the highlighted part would be written
if the file was modified and the user answers yes to the "Save?"
prompt.

This fixes https://savannah.gnu.org/bugs/?52474.

src/text.c

@@ -3068,6 +3068,9 @@ void do_linter(void)
 	return;
     }
 
+    openfile->mark_set = FALSE;
+    edit_refresh();
+
     if (openfile->modified) {
 	int i = do_yesno_prompt(FALSE, _("Save modified buffer before linting?"));