Fix(GraphQL): Ignore redundant references to inverse object in mutation rewriting

[vmrajas]

After the refactoring of mutation rewriting, references to inverse objects were not ignored in existenceQueries function. This PR fixes this.

This PR fixes https://discuss.dgraph.io/t/bug-not-sure-how-to-name-this-bug-please-help-out/12845
Testing:

1. Basic yaml tests

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This change is 

[JatinDev543]

Reviewable status: 0 of 3 files reviewed, 1 unresolved discussion (waiting on @abhimanyusinghgaur, @pawanrawal, and @vmrajas)

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graphql/resolve/add_mutation_test.yaml, line 4170 at r1 (raw file):

            "dgraph.type":["Foo"],
            "uid":"_:Foo3"
        }

can you please add test(if not already present) where we have nested fields but they are not inverse.
For example with below schema
type Foo {
id: String! @id
bar: Bar!
}
type Bar {
id: String! @id
foo: Foo!
}

mutation addBar($input: [AddBarInput!]!) {
  addBar(input: $input) {
    bar {
      id
      foo {
        id
        bar{
          id
        
        }  
      }
    }
  }
}

[JatinDev543]

Reviewed 3 of 3 files at r1.
Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @pawanrawal and @vmrajas)

[vmrajas]

Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @jatindevdg and @pawanrawal)

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graphql/resolve/add_mutation_test.yaml, line 4170 at r1 (raw file):

Previously, JatinDevDG (Jatin Dev) wrote…

can you please add test(if not already present) where we have nested fields but they are not inverse.
For example with below schema
type Foo {
id: String! @id
bar: Bar!
}
type Bar {
id: String! @id
foo: Foo!
}

mutation addBar($input: [AddBarInput!]!) {
  addBar(input: $input) {
    bar {
      id
      foo {
        id
        bar{
          id
        
        }  
      }
    }
  }
}

There exists such a test in add_mutation_test.yaml by the name, "Deep mutation three level xid with no initial XID" .

It adds Post1 ->linking it to -> Comment1 -> linking it to another Comment1 as reply .There are no inverses.