revlog: faster hash computation when one of the parent node is null Because we often compute sha1(nullid), it's interesting to copy a precomputed hash of nullid instead of computing everytime the same hash. Similarly, when one of the parents is null, we can avoid a < comparison (sort). Overall, this change adds a string equality comparison on each hash() call, but when p2 is null, we drop one string < comparison, and copy a hash instead of computing it. Since it is common to have revisions with only one parent, this change makes hash() 25% faster when cloning a big repository.