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.
