irmin-pack: fix a bug with encoding of new inode values
irmin-pack: fix a bug with encoding of new inode values

The previous implementation mistakenly uses the hash of the _parent_
when encoding a pointer to an unindexed child.

Spotted by the Inode encoding tests introduced by
https://github.com/mirage/irmin/pull/1633.

mirage/irmin,mirage/irmin