Fixing a small logic bug in diffList

It’s not enough to only check if no new value is set. It can also be
that a new value is set, but contains a variable that cannot be
interpolated until a depending resource is created during the apply
fase.

I actually found this one as one of the acceptance tests for the AWS
ELB resource was failing. It failed with the following error:

```
--- FAIL: TestAccAWSELB_InstanceAttaching (177.83 seconds)
  testing.go:121: Step 1 error: Error applying: aws_elb.bar: diffs
didn't match during apply. This is a bug with the resource provider,
please report a bug.
FAIL
exit status 1
FAIL  github.com/hashicorp/terraform/builtin/providers/aws  177.882s
```

After a quick look I noticed it was actually a bug in core TF so added
the test and made sure all unit tests and AWS acceptance tests are now
running successfully.

helper/schema/schema.go

@@ -459,9 +459,10 @@ func (m schemaMap) diffList(
 	o, n, _, computedList := d.diffChange(k)
 	nSet := n != nil
 
-	// If we have an old value, but no new value set but we're computed,
-	// then nothing has changed.
-	if o != nil && n == nil && schema.Computed {
+	// If we have an old value and no new value is set or will be
+	// computed once all variables can be interpolated and we're
+	// computed, then nothing has changed.
+	if o != nil && n == nil && !computedList && schema.Computed {
 		return nil
 	}
 

helper/schema/schema_test.go

@@ -1466,6 +1466,57 @@ func TestSchemaMap_Diff(t *testing.T) {
 
 			Err: false,
 		},
+
+		{
+			Schema: map[string]*Schema{
+				"internal": &Schema{
+					Type:     TypeBool,
+					Required: true,
+				},
+
+				"instances": &Schema{
+					Type:     TypeSet,
+					Elem:     &Schema{Type: TypeString},
+					Optional: true,
+					Computed: true,
+					Set: func(v interface{}) int {
+						return len(v)
+					},
+				},
+			},
+
+			State: &terraform.InstanceState{
+				Attributes: map[string]string{
+					"internal":    "false",
+					"instances.#": "0",
+				},
+			},
+
+			Config: map[string]interface{}{
+				"internal":  true,
+				"instances": []interface{}{"${var.foo}"},
+			},
+
+			ConfigVariables: map[string]string{
+				"var.foo": config.UnknownVariableValue,
+			},
+
+			Diff: &terraform.InstanceDiff{
+				Attributes: map[string]*terraform.ResourceAttrDiff{
+					"internal": &terraform.ResourceAttrDiff{
+						Old: "0",
+						New: "1",
+					},
+
+					"instances.#": &terraform.ResourceAttrDiff{
+						Old:         "0",
+						NewComputed: true,
+					},
+				},
+			},
+
+			Err: false,
+		},
 	}
 
 	for i, tc := range cases {