[Rpm-maint] [rpm-software-management/rpm] Please support smaller build-ids (#950)

[Rafael Ávila de Espíndola]

This is the birthday problem, so a good 64 bit hash needs about 32 bit worth of hashes for a collision. Note that there is precedent for using only 64 bits. The dwarf standard (http://www.dwarfstd.org/doc/DWARF5.pdf section 7.32  "Type Signature Computation") uses the last 8 bytes of md5 for type signatures.

As for the quality, I think xxhash is very good (http://cyan4973.github.io/xxHash/) , but I don't have any detailed knowledge of hashes.

If we really need 128 bits, which hash would you suggest for a fast buildid? Farmhash comes to mind.

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