cli: Fix diff for nested set unchanged elements

Unchanged elements in nested attributes backed by sets were previously
misrendered as empty objects. This commit removes the additional
brackets and adds a count of unchanged elements.

internal/command/format/diff.go

@@ -569,6 +569,8 @@ func (p *blockBodyDiffPrinter) writeNestedAttrDiff(
 
 		p.buf.WriteString(" = [")
 
+		var unchanged int
+
 		for it := all.ElementIterator(); it.Next(); {
 			_, val := it.Element()
 			var action plans.Action
@@ -598,12 +600,17 @@ func (p *blockBodyDiffPrinter) writeNestedAttrDiff(
 				newValue = val
 			}
 
+			if action == plans.NoOp {
+				unchanged++
+				continue
+			}
+
 			p.buf.WriteString("\n")
 			p.buf.WriteString(strings.Repeat(" ", indent+4))
 			p.writeActionSymbol(action)
 			p.buf.WriteString("{")
 
-			if action != plans.NoOp && (p.pathForcesNewResource(path) || p.pathForcesNewResource(path[:len(path)-1])) {
+			if p.pathForcesNewResource(path) || p.pathForcesNewResource(path[:len(path)-1]) {
 				p.buf.WriteString(p.color.Color(forcesNewResourceCaption))
 			}
 
@@ -615,6 +622,7 @@ func (p *blockBodyDiffPrinter) writeNestedAttrDiff(
 			p.buf.WriteString("},")
 		}
 		p.buf.WriteString("\n")
+		p.writeSkippedElems(unchanged, indent+4)
 		p.buf.WriteString(strings.Repeat(" ", indent+2))
 		p.buf.WriteString("]")
 

internal/command/format/diff_test.go

@@ -2822,6 +2822,10 @@ func TestResourceChange_nestedSet(t *testing.T) {
 						"mount_point": cty.StringVal("/var/diska"),
 						"size":        cty.NullVal(cty.String),
 					}),
+					cty.ObjectVal(map[string]cty.Value{
+						"mount_point": cty.StringVal("/var/diskb"),
+						"size":        cty.StringVal("100GB"),
+					}),
 				}),
 				"root_block_device": cty.SetVal([]cty.Value{
 					cty.ObjectVal(map[string]cty.Value{
@@ -2838,6 +2842,10 @@ func TestResourceChange_nestedSet(t *testing.T) {
 						"mount_point": cty.StringVal("/var/diska"),
 						"size":        cty.StringVal("50GB"),
 					}),
+					cty.ObjectVal(map[string]cty.Value{
+						"mount_point": cty.StringVal("/var/diskb"),
+						"size":        cty.StringVal("100GB"),
+					}),
 				}),
 				"root_block_device": cty.SetVal([]cty.Value{
 					cty.ObjectVal(map[string]cty.Value{
@@ -2859,6 +2867,7 @@ func TestResourceChange_nestedSet(t *testing.T) {
           - {
             - mount_point = "/var/diska" -> null
           },
+          # (1 unchanged element hidden)
         ]
         id    = "i-02ae66f368e8518a9"