cli: Fix diff for nested set unchanged elements

[alisdair]

Unchanged elements in nested attributes backed by sets were previously misrendered as empty objects. This PR removes the additional brackets and adds a count of unchanged elements, and secondarily fixes the indentation for these diffs.

Targeting 1.1 backport.

Screenshots

Before:

After:

[apparentlymart]

Nice! Thanks for digging into this.

---

Looking at your "after" screenshot it still felt a little "off", though it did take me staring at it for quite a few minutes before I realized what was throwing me:

It seems like all of the lines after the opening - { are missing one level of indentation, so the nested -s are directly below the brace rather than indented relative to it.

While I guess it's a matter of opinion whether it's a "good thing", in all other cases we have created the effect of each additional nesting level adding four columns of indent and then having the change symbols ~ - + live inside the left "gutter" when they are present, as if they are annotations in the margin rather than part of the actual content. Notice for example how the closing ] for ~ values = [ lines up with values rather than with ~.

It'd be totally reasonable to treat that as out of scope for what you were aiming to solve here, so I'm not intending this as a blocker for merging, but if it's something you can see an easy path to fix while you're there it'd be nice to make this consistent with the formatting elsewhere.

[apparentlymart]

GitHub won't let me properly bracket the unchanged lines here to make this a real "suggestion" but here's a proposed alternative expected output for the indentation I was talking about in my review comment:

  # test_instance.example will be updated in-place
  ~ resource "test_instance" "example" {
      ~ ami   = "ami-BEFORE" -> "ami-AFTER"
      ~ disks = [
          + {
              + mount_point = "/var/diska"
              + size        = "50GB"
            },
          - {
              - mount_point = "/var/diska" -> null
            },
          # (1 unchanged element hidden)
        ]
        id    = "i-02ae66f368e8518a9"

      + root_block_device {
          + new_field   = "new_value"
          + volume_type = "gp2"
        }
      - root_block_device {
          - volume_type = "gp2" -> null
        }
    }

(That was kinda awkward to edit in the GitHub comment box so I probably didn't get it all exactly right, but the indentation of the disks contents is the main thing I'm getting at here.)

[alisdair]

Great catch! The indentation fix seems pretty simple to me, so I've rolled it into this PR. Description & screenshot updated too.

I also quickly checked that the other nested attribute types are indented correctly, and to me they look right:

[apparentlymart]

Thanks!

The second example in your screenshot still seems a little off: the + number = 5 line has the + directly under the " of "baz" rather than indented (under the a), and the closing }, is directly under the ~ rather than under the " of "baz".

With that said, still potentially out of scope for what you're doing here so would be totally reasonable to just open a new issue to represent that and fix it up another time, but I'm hoping 🤞 that the pretty simple fix you found for the tuple/list case is also pretty simple for the object/map case. 😁

[alisdair]

🤦 Thanks for catching that too. I had already merged this PR so I've opened #29986 to hopefully address it. The same issue looked to be the case for the nested single value, so I adjusted that too.

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