command/format: Fix rendering of unknown elements in set/map/list

[radeksimko]

Fixes #19998

The diff we produce may not be the best (see test cases), but the definition of "best" is kind of subjective here.

AFAIK 0.11 used to render [a, b, c] -> [a, (unknown), c] as

[
  a,
~ b -> (unknown),
  c,
]

but we now render it as

[
  a,
- b,
+ (unknown),
  c,
]

neither of which is not necessarily right or wrong. LCS is however most likely better than simple index-based comparison.

[hashicorp-cla]

All committers have signed the CLA.

[apparentlymart]

LGTM, thanks!

(Remember to complete the CLA process before merging this.)

[ghost]

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