parentdelta: compress.py extension is no longer necessary
Using `hg --config format.parentdelta=True clone --pull src dst` is equivalent.
Ancestors of 54 2 0 Ancestors of 6 and 53 4 2 1 0 Ancestors of 5 and 44 2 0 Descendants of 57 8 Descendants of 5 and 36 7 8 Descendants of 5 and 45 7 8