Matches https://github.com/ilyagr/pijul-clone as of now, might become out of sync

Fix the hunk_roundtrip test, simplify code, improve test coverage, fix some edge cases.

Created by  FHRXP5Jnb2MWLDrPrnLnkN2ryWcGCo6CRr1dXR9FW2YA  on December 9, 2021
YQMLICLW2FABJBX5AQGPZIHICYLECR53TLPEBGUEH23YNKIMKC7AC
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