auto-upgrade: introduce a way to auto-upgrade to/from share-safe This is the first "automatic-upgrade" capability. In the following commits, similar features are coming for other "fast to upgrade" formats. This is different from the `safe-mismatch.source-not-safe` and `safe-mismatch.source-safe` configuration that deal with mismatch between a share and its share-source. Here we are dealing with mismatch between a repository configuration and its actual format. We will need further work for cases were the repository cannot be locked. A basic protection is in place to avoid a infinite loop for now, but it will get proper attention in a later changeset. Differential Revision: https://phab.mercurial-scm.org/D12611