cli: Optimize for large multi-line string outputs

[alisdair]

When rendering a new, very large multi-line string output as part of terraform plan or terraform apply, we hit this code path:

terraform/command/format/diff.go

Lines 699 to 711 in 14936e6

	if strings.Contains(val.AsString(), "\n") {
	// It's a multi-line string, so we want to use the multi-line
	// rendering so it'll be readable. Rather than re-implement
	// that here, we'll just re-use the multi-line string diff
	// printer with no changes, which ends up producing the
	// result we want here.
	// The path argument is nil because we don't track path
	// information into strings and we know that a string can't
	// have any indices or attributes that might need to be marked
	// as (requires replacement), which is what that argument is for.
	p.writeValueDiff(val, val, indent, nil)
	break
	}

The writeValueDiff multi-line string renderer calculates a longest common subsequence, which is an operation with quadratic time and space complexity. When the "two" strings are in fact the same value (as you can see from the call above), this results in a worst-case execution time. For very large strings, this can be several minutes:

$ time terraform-0.14.5 plan >/dev/null

real	4m3.381s
user	3m53.593s
sys	0m15.892s

This commit adds an early shortcut for this specific case, using simple string equality as the test, which directly prints the multi-line string without calculating an LCS. This dramatically improves the runtime in this specific edge case:

$ time terraform plan > /dev/null

real	0m0.104s
user	0m0.123s
sys	0m0.043s

Fixes #27743

[codecov]

Codecov Report

Merging #27746 (943df48) into master (14936e6) will decrease coverage by 0.00%.
The diff coverage is 87.50%.

Impacted Files	Coverage Δ
command/format/diff.go	90.63% <87.50%> (+0.05%)	⬆️
internal/providercache/dir.go	67.34% <0.00%> (-6.13%)	⬇️
dag/marshal.go	61.90% <0.00%> (-1.59%)	⬇️
terraform/evaluate.go	52.47% <0.00%> (-0.42%)	⬇️
states/statefile/version4.go	57.96% <0.00%> (-0.24%)	⬇️
terraform/node_resource_plan.go	98.05% <0.00%> (+1.94%)	⬆️

[ghost]

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