upgrade: fix handling of `.d` data file The test did not used any `.d` file. So of course, their handling was broken (for vfs encoding reason again). We fix the issue and update the test. Note that some fncache issue remains with `.d` file. It will be taken care of in the next changesets. Differential Revision: https://phab.mercurial-scm.org/D6904

upgrade: also copy data file Differential Revision: https://phab.mercurial-scm.org/D6903

upgrade: create the correct destination directory for copies revlogs The encoding in vfs mess up with the directory we are trying to create manually. We could duplicate the advanced logic some more, but that seems ill fated. Instead, we let the vfs deal with directory creation. We update the test to contains a name affected by encoding. Differential Revision: https://phab.mercurial-scm.org/D6902

upgrade: fix DELTAREUSEFULLADD implementation in revlog.clone If we do a full addition, we need to start from the full text. Differential Revision: https://phab.mercurial-scm.org/D6901

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