requirements: add constant for revlog v1 requirement Since this series is adding a new requirement, we might as well clean up while we're here. Differential Revision: https://phab.mercurial-scm.org/D10105

error: add `hint` attribute to `SidedataHashError` This prevents an exception within an exception because `hint` does not exist. Differential Revision: https://phab.mercurial-scm.org/D10024

changegroup: use the local variable instead of reaching through self Gratuitous cleanup while I was here. Differential Revision: https://phab.mercurial-scm.org/D10023

config: use level to properly deal with value priority A higher priority alias will now take precedence over lower priority ones. This was a requirements step before using alias more widely, especially to rename existing and established config option. Differential Revision: https://phab.mercurial-scm.org/D9927

config: track the "level" of a value Config value now remember the "level" of the config that loaded it. This will be used to ensure good priority management for alias. Differential Revision: https://phab.mercurial-scm.org/D9926

config: track "source" along side value Currently the source is stored in a entirely different way than the data. This is impractical. Especially if we are about to add more of such metadata. So lets put them back together. Differential Revision: https://phab.mercurial-scm.org/D9925

config: use a new `alter` method in `fixconfig` The `set` function is doing various work related to metadata (eg: the source, later the level). However the `fixconfig` call only updating some values according to standard processing, not changing any of the related metadata. So we introduce a new method and use it there. Differential Revision: https://phab.mercurial-scm.org/D9924