upgrade: document DELTAREUSEFULLADD in revlog.clone The documentation of the `debugupgraderepo` command was updated when `re-delta-fulladd` was introduced. However, the docstring of the `revlog.clone` method was not. Differential Revision: https://phab.mercurial-scm.org/D6900

upgrade: move most of revlog.clone method into a _clone method The content of the clone method now focus on parameters validation and processing. The `_clone` method focus on the actual cloning logic. Splitting the method out save some indentation and clarify each method code since it a focussed on one goal. Differential Revision: https://phab.mercurial-scm.org/D6899

flagprocessors: remove flagprocessorsmixin It became an empty shell. Differential Revision: https://phab.mercurial-scm.org/D6823

flagprocessors: move _flagserrorclass attribute on revlog & co This is a small duplication, and the last bit we need to get rid of the mixin. Honestly, I am not fan of that class attribute and it mostly exist to accomodate The simple-storage whose usage of flag processors is dumbious and that is currently dead code anyway. However I don't want to be pulled into futher unrelated cleaning so it is a small price to pay. Differential Revision: https://phab.mercurial-scm.org/D6822

flagprocessors: directly duplicate the deprecated layer back into revlog The code duplication benign and will get removed in a couple of month anyway. Differential Revision: https://phab.mercurial-scm.org/D6821

flagprocessors: make `processflagsraw` a module level function One more steps toward removing the mixin. Differential Revision: https://phab.mercurial-scm.org/D6820