Let cancel after an unambiguous completion was accepted undo it

In some cases the completion we come up with may be unexpected, e.g.
if you have files like

/etc/realfile

and

/etc/wrongfile

and enter "/etc/gile", it will accept "wrongfile" because "g" and
"ile" are in there - it's a substring insertion match.

The underlying cause was a typo, so it should be easy to go back.

So we do a bit of magic and let "cancel" undo, but only right after a
completion was accepted via complete or complete-and-search.

That means that just reflexively pressing escape would, by default, get you back to
the old token and let you fix your mistake.

We don't do this when the completion was accepted via the pager,
because 1. there's more of a chance to see the problem there and 2.
it's harder to redo in that case.

Fixes #7433.

src/reader.cpp

@@ -2790,7 +2790,19 @@ void reader_data_t::handle_readline_command(readline_cmd_t c, readline_loop_stat
             break;
         }
         case rl::cancel: {
-            // The only thing we can cancel right now is paging, which we handled up above.
+            // If we last inserted a completion, undo it.
+            // This doesn't apply if the completion was selected via the pager
+            // (in which case the last command is "execute" or similar,
+            // but never complete{,_and_search})
+            //
+            // Also paging is already cancelled above.
+            if (rls.complete_did_insert &&
+                (rls.last_cmd == rl::complete
+                 || rls.last_cmd == rl::complete_and_search)) {
+                editable_line_t *el = active_edit_line();
+                el->undo();
+                update_buff_pos(el);
+            }
             break;
         }
         case rl::repaint_mode: {