branchmap: Save changectx creation during update The newly introduced `branchmap` function allows us to skip the creation of changectx objects. This speeds up the construction of the branchmap. On the mozilla repository (117293 changesets, 15490 mutable) Before: ! impactable 19.9 ! mutable 0.576 ! unserved 3.16 After: ! impactable 7.03 (2.8x faster) ! mutable 0.352 (1.6x) ! unserved 1.15 (2.7x) On the cpython repository (81418 changesets, 6418 mutable) Before: ! impactable 15.9 ! mutable 0.451 ! unserved 0.861 After: ! impactable 6.55 (2.4x faster) ! mutable 0.170 (2.6x faster) ! unserved 0.289 (2.9x faster) On the pypy repository (58852 changesets) Before: ! impactable 13.6 After: ! impactable 6.17 (2.2x faster) On my Mercurial repository (18295 changesets, 2210 mutable) Before: ! impactable 23.9 ! mutable 0.368 ! unserved 0.057 After: ! impactable 1.31 (18x faster) ! mutable 0.042 (8.7x) ! unserved 0.025 (2.2x)