renderdag: emit link_line for same-column named parents

[geraschenko]

Fixes #1292

The actual fix is the 3-line change to eden/scm/lib/renderdag/src/render.rs. Everything else is adding a new test fixture (BASIC_DISCONNECTED) and test (basic_disconnected_min_row_height_1) which demonstrates the issue.

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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[quark-zju]

Hi, I think it's better to put a blank line instead adding an vertical edge:

o  C 
o  B

o  A

The format (no vertical lines) is inherited from hg's "experimental.graphshorten=True" feature, which uses this dense format:

  $ sl log -G -T '{rev}\n' --config 'experimental.graphshorten=True'
  @  9
  o  8
  │ o  7
  │ o  6
  ╭─┤
  │ o  5
  o │  4
  │ o  3
  o │  2
  ├─╯
  o  1
  o  0

Could you change it to use blank line?

[meta-codesync]

This pull request has been imported. If you are a Meta employee, you can view this in D106722405. (Because this pull request was imported automatically, there will not be any future comments.)

[facebook-github-tools]

@geraschenko has updated the pull request. You must reimport the pull request before landing.

[geraschenko]

I've updated the PR based on the discussion on discord (starting at https://discord.com/channels/1042527964224557107/1042527965256364157/1509570566812598463). There are two modes for indicating disconnection, using either vertical space (default, a blank line is added when consecutive nodes in the same column are not connected) or horizontal space (when stagger_disconnected_nodes is true, consecutive disconnected nodes are not allowed to be in the same column).

[meta-codesync]

@quark-zju merged this pull request in 658d950.