ancestor: add lazy membership testing to lazyancestors This also makes the perfancestorset command use lazy membership testing. In a linear repository with over 400,000 commits, without this patch, hg perfancestorset takes 0.80 seconds no matter how far behind we're looking. With this patch, hg perfancestorset -- X takes: Rev X Time -1 0.00s -4000 0.01s -20000 0.04s -80000 0.17s -200000 0.43s -300000 0.69s 0 0.88s Thus, for revisions close to tip, we're up to several orders of magnitude faster. At 0 we're around 10% slower.