visually hint how deep a path is in `at` style

[FelipeLema]

When using at style I need to be aware (alert, even) of when a path is going to be done.

This visual hint will help me (and hopefully other people) queue keyboard input for next actions.

• "ok, so this jump takes one key, so once I jump I can copy-a-line, then... "
• "oh, I'll have to input one extra key after this..."