fix(DGRAPH): fix @normalize response when multiple fields at different levels with same alias are selected.

[JatinDev543]

We are generating incorrect order of the fields in a result of the @normalize directive when there are multiple fields with same Alias at nested levels. As a result of that in resulting JSON, the nodes with the same Alias were not converted to list.
For example, if we have below data in dgraph .

{
      "_id": "Process",
      "Name": "Process",
      "Extends": {
        "_id": "Core/Namespace",
        "Name": "Namespace"        
      }
}

And if we do below query with @recurse and @normalize directive

{
  item as var(func:eq(_id,"Process"))
  b2(func:uid(item))@normalize@recurse {
    Name:Name id:_id  Extends
  }
}

We will get this response from Dgraph where fields with same alias are not together and while converting response to JSON , we are not able to combine fields with same Alias in a list.

"b2": [
      {
        "Name": "Process",
        "id": "Process",
        "Name":"Namespace",
        "id": "Core/Namespace"
      }
    ]

In this PR we fix this in normalize, by putting all fields with the same Alias together. So now response of above query will be

"b2": [
      {
        "Name": [
          "Process",
          "Namespace"
        ],
        "id": [
          "Process",
          "Core/Namespace"
        ]
      }
]

This behaviour is broken by PR #6362. Prior to this , we used sorting on fields which guarantee that fields with same name or Alias comes together.

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This change is 

[NamanJain8]

Looks good. 🎉
Some minor nit-picks.

[manishrjain]

Got a few comments. Thanks @NamanJain8 for the review.

Reviewed 1 of 2 files at r2, 1 of 1 files at r4.
Reviewable status: all files reviewed, 7 unresolved discussions (waiting on @jatindevdg, @NamanJain8, and @pawanrawal)

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query/outputnode.go, line 156 at r4 (raw file):

func (enc *encoder) MergeSort(headRef *fastJsonNode) {
	head := *headRef
	var a fastJsonNode

move this closer to usage.

---

query/outputnode.go, line 158 at r4 (raw file):

	var a fastJsonNode
	var b fastJsonNode

Remove unnecessary vertical spaces.

---

query/outputnode.go, line 171 at r4 (raw file):

}

func (enc *encoder) sortedMerge(a fastJsonNode, b fastJsonNode) fastJsonNode {

Rename to mergeSortedLists

[JatinDev543]

Reviewable status: 1 of 2 files reviewed, 7 unresolved discussions (waiting on @manishrjain, @NamanJain8, and @pawanrawal)

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query/outputnode.go, line 155 at r3 (raw file):

Previously, JatinDevDG (Jatin Dev) wrote…

done

done.

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query/outputnode.go, line 194 at r3 (raw file):

Previously, NamanJain8 (Naman Jain) wrote…

return strings.Compare(enc.attrForID(attri), enc.attrForID(attrj)) <= 0

done.

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query/outputnode.go, line 958 at r3 (raw file):

Previously, JatinDevDG (Jatin Dev) wrote…

done.

done.

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query/outputnode.go, line 156 at r4 (raw file):

Previously, manishrjain (Manish R Jain) wrote…

move this closer to usage.

moved.

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query/outputnode.go, line 158 at r4 (raw file):

Previously, manishrjain (Manish R Jain) wrote…

Remove unnecessary vertical spaces.

removed.

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query/outputnode.go, line 171 at r4 (raw file):

Previously, manishrjain (Manish R Jain) wrote…

Rename to mergeSortedLists

renamed.