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)

changelog: add a `branch` method, bypassing changectx The only way to access the branch of a changeset is currently to create a changectx object and access its `branch()` method. Creating a new Python object is costly and has a huge impact on code doing heavy access to `branch()` (like branchmap). This change introduces a new method on changelog that allows direct access to the branch of a revision. See the next changeset for impact.

branchmap: pass revision insteads of changectx to the update function Creation of changectx objects is very slow, and they are not very useful. We are going to drop them. The first step is to change the function argument type.