Fix diff alignment and remove no change DC

Old Output:
```
+/- Job: "example"
    Datacenters {
  Datacenters: "dc1"
}
+/- Task Group: "cache" (1 create/destroy update)
  +/- RestartPolicy {
    +/- Attempts: "10" => "9"
        Delay:    "25000000000"
        Interval: "300000000000"
        Mode:     "delay"
  }
  +/- EphemeralDisk {
        Migrate: "false"
    +/- SizeMB:  "300" => "301"
        Sticky:  "false"
  }
  +/- Task: "redis" (forces create/destroy update)
    +   Meta[key]: "value"
    +/- Config {
          image:           "redis:3.2"
      +/- port_map[0][db]: "6379" => "6380"
    }
    +/- Resources {
          CPU:      "500"
          DiskMB:   "0"
          IOPS:     "0"
      +/- MemoryMB: "256" => "257"
    }
    +/- Service {
          Name:      "global-redis-check"
          PortLabel: "db"
      +/- Check {
            Command:       ""
            InitialStatus: ""
            Interval:      "10000000000"
            Name:          "alive"
            Path:          ""
            PortLabel:     ""
            Protocol:      ""
        +/- Timeout:       "2000000000" => "3000000000"
            Type:          "tcp"
      }
    }
```

New Output:
```
+/- Job: "example"
+/- Task Group: "cache" (1 create/destroy update)
  +/- RestartPolicy {
    +/- Attempts: "10" => "9"
        Delay:    "25000000000"
        Interval: "300000000000"
        Mode:     "delay"
      }
  +/- EphemeralDisk {
        Migrate: "false"
    +/- SizeMB:  "300" => "301"
        Sticky:  "false"
      }
  +/- Task: "redis" (forces create/destroy update)
    +   Meta[key]: "value"
    +/- Config {
          image:           "redis:3.2"
      +/- port_map[0][db]: "6379" => "6380"
        }
    +/- Resources {
          CPU:      "500"
          DiskMB:   "0"
          IOPS:     "0"
      +/- MemoryMB: "256" => "257"
        }
    +/- Service {
          Name:      "global-redis-check"
          PortLabel: "db"
      +/- Check {
            Command:       ""
            InitialStatus: ""
            Interval:      "10000000000"
            Name:          "alive"
            Path:          ""
            PortLabel:     ""
            Protocol:      ""
        +/- Timeout:       "2000000000" => "3000000000"
            Type:          "tcp"
          }
        }
```

nomad/structs/diff.go

@@ -98,7 +98,7 @@ func (j *Job) Diff(other *Job, contextual bool) (*JobDiff, error) {
 	diff.Fields = fieldDiffs(oldPrimitiveFlat, newPrimitiveFlat, false)
 
 	// Datacenters diff
-	if setDiff := stringSetDiff(j.Datacenters, other.Datacenters, "Datacenters", false); setDiff != nil {
+	if setDiff := stringSetDiff(j.Datacenters, other.Datacenters, "Datacenters", contextual); setDiff != nil && setDiff.Type != DiffTypeNone {
 		diff.Objects = append(diff.Objects, setDiff)
 	}
 

nomad/structs/diff_test.go

@@ -397,7 +397,7 @@ func TestJobDiff(t *testing.T) {
 			},
 		},
 		{
-			// Datacenter contextual
+			// Datacenter contextual no change
 			Contextual: true,
 			Old: &Job{
 				Datacenters: []string{"foo", "bar"},
@@ -409,6 +409,45 @@ func TestJobDiff(t *testing.T) {
 				Type: DiffTypeNone,
 			},
 		},
+		{
+			// Datacenter contextual
+			Contextual: true,
+			Old: &Job{
+				Datacenters: []string{"foo", "bar"},
+			},
+			New: &Job{
+				Datacenters: []string{"foo", "bar", "baz"},
+			},
+			Expected: &JobDiff{
+				Type: DiffTypeEdited,
+				Objects: []*ObjectDiff{
+					{
+						Type: DiffTypeAdded,
+						Name: "Datacenters",
+						Fields: []*FieldDiff{
+							{
+								Type: DiffTypeAdded,
+								Name: "Datacenters",
+								Old:  "",
+								New:  "baz",
+							},
+							{
+								Type: DiffTypeNone,
+								Name: "Datacenters",
+								Old:  "bar",
+								New:  "bar",
+							},
+							{
+								Type: DiffTypeNone,
+								Name: "Datacenters",
+								Old:  "foo",
+								New:  "foo",
+							},
+						},
+					},
+				},
+			},
+		},
 		{
 			// Update strategy edited
 			Old: &Job{