Remove the use of XOR hashing in extraHash computation

[Friendseeker]

zinc/internal/zinc-core/src/main/scala/sbt/internal/inc/Incremental.scala

Lines 883 to 889 in 1899979

	val analysis = handler.previousAnalysis
	val externalParents = analysis.relations.inheritance.external.forward(className)
	val internalParents = analysis.relations.inheritance.internal.forward(className)
	val externalParentsAPI = externalParents.map(analysis.apis.externalAPI)
	val internalParentsAPI = internalParents.map(analysis.apis.internalAPI)
	val parentsHashes = (externalParentsAPI ++ internalParentsAPI).map(_.extraHash())
	parentsHashes.fold(currentExtraHash)(_ ^ _)

We use parentsHashes.fold(currentExtraHash)(_ ^ _) which is a bad way to combine hashes together. If we use XOR to combine different hashes, identical hashes gets cancelled out. Since we are doing hierarchical hashing here I have a concern that such cancellation would happen.

Consider to replace with parentsHashes.hashCode instead.