renderdag: emit link_line for same-column named parents

[geraschenko]

Fixes #1292. With min_row_height(1), connected and disconnected same-column adjacent nodes render identically, with no │ between connected nodes.

Before / After (box_drawing, min_row_height=1) for new BASIC_DISCONNECTED fixture (dag: "A B-C", heads ["A", "C"]):

Before:
o  C
o  B
o  A

After:
o  C
│
o  B
o  A

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