copies: avoid unwanted side effect from one branch to another Without this copy, change in a one descendant branch (With "remove" change only) could affect computation on another descendant branches. This was not caugh by the test because the test graph are "too simple". I started writing more test in that regards, but I a submitting this changes earlier because I want to get more code landed to allow other optimisation work to happens. Differential Revision: https://phab.mercurial-scm.org/D9416

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