bundle2: now that we have a seek implementation, use it Replace bare part.read() calls with part.seek(0, 2) since the return value is being ignored. As this doesn't necessarily require building a string that contains the rest of the part, the potential exists to reduce the memory footprint of these operations.

obsolete: populate successors, precursors, children lazily The precursors and children dictionaries are not used by many commands. By making them lazily populated, 'hg log -r @~10::@ >/dev/null' is sped up from 0.564s to 0.440s on my hg.hg repo with 73k markers. Also make successors lazily populated, mostly for consistency with the others.

obsolete: pass only new markers to _checkinvalidmarkers() We will soon delay populating precursors until we have to. To prepare for that, make _checkinvalidmarkers() scan for a nullid precursor in the list of new markers instead of the (currently) cheaper 'if nullid in precursors' check.