Fix overly strict backward_sorted_until

A change that ended exactly where the previous one started was not
considered backward sorted. Leading to some very bad performances in
certain cases, like '100000o<esc>u'

src/changes.cc

@@ -86,7 +86,7 @@ const Buffer::Change* backward_sorted_until(const Buffer::Change* first, const B
     if (first != last) {
         const Buffer::Change* next = first;
         while (++next != last) {
-            if (first->begin <= next->end)
+            if (first->begin < next->end)
                 return next;
             first = next;
         }