revset: add optimization for heads(commonancestors()) Previously, the only way to get these commits were (tested on mozilla-central): hg perfrevset 'heads(::a7cf55 and ::d8b15)' ! wall 4.988366 comb 4.960000 user 4.780000 sys 0.180000 (best of 3) After this patch: (python) hg perfrevset 'heads(commonancestors(a7cf55 + d8b15))' ! wall 0.002155 comb 0.000000 user 0.000000 sys 0.000000 (best of 1107) (C) hg perfrevset 'heads(commonancestors(a7cf55 + d8b15))' ! wall 0.000568 comb 0.000000 user 0.000000 sys 0.000000 (best of 4646)