pack-objects: replace name_hash with min_hash

Hash the content of objects to group by similar content.
* 8 byte sliding window Adler-like hash
* Mix the 32 bit hash with murmur3
* Take minimum and maximum hash values
* Take the lower 16 bits of these and concatenate

First and final steps of Adler and murmur3 respectively merged.
Low bits of Adler replaced with multiplicative hash, base 3.