fix(OverlappingFieldsCanMerge): fix var scope for fragment comparison cache

[yaacovCR]

Previously, OverlappingFieldsCanMerge mistakenly used getFragmentSignature() while visiting a SelectionSet. getFragmentSignature() is a helper only populated while visiting a FragmentSpread to assist with argument validation. This mistake led to processing of selection sets inside fragment definitions without their fragment variable scope.

Without the scope, the comparison cache confuses identical-looking spreads from a fragment definition and an operation. After checking the fragment, OverlappingFieldsCanMerge would incorrectly skip the operation's separate conflict. For example:

    fragment Outer($x: Int, $y: Int) on Type {
      ...WithArgs(x: $x)
      ...WithArgs(x: $y)
    }

    fragment WithArgs($x: Int) on Type {
      a(x: $x)
    }

    query Example($x: Int, $y: Int) {
      ...WithArgs(x: $x)
      ...WithArgs(x: $y)
    }

Previously, the conflict in Outer was reported but the conflict in Example was not. This PR fixes that mistake by using getFragmentSignatureByName() while entering the enclosing FragmentDefinition and saving the current fragment.

[vercel]

@yaacovCR is attempting to deploy a commit to the The GraphQL Foundation Team on Vercel.

A member of the Team first needs to authorize it.