discovery: compute newly discovered missing in a more efficient way Calling "descendants" is expensive, instead, we bound the walk inside the know set of undecided revision. This help with discovery performance: # without the revset '%ld' improvement $ hg perfdiscovery -R pypy-left pypy-right before: wall 0.675631 comb 0.680000 user 0.670000 sys 0.010000 (median of 15) after: wall 0.520145 comb 0.530000 user 0.510000 sys 0.020000 (median of 19) There is another series in flight that greatly improves performances of "%ld" substitution in `repo.revs` call. If this changeset is applied above it, we see a similar performance boost. # with the revset '%ld' improvement $ hg perfdiscovery -R pypy-left pypy-right before: wall 0.477848 comb 0.480000 user 0.480000 sys 0.000000 (median of 22) after: wall 0.404163 comb 0.400000 user 0.400000 sys 0.000000 (median of 24)