[1.x] Use hashCode instead of XOR hash in extraHash computation

[Friendseeker]

Issue

From #1399

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.

Also nowhere else in Zinc codebase do we use XOR to combine hashes, so there is also a consistency issue.

Fix

Replace XOR with hashCode.

[Friendseeker]

I guess this is not a trivial change after all... CIs are all failing

Shall investigate further