share: move share safe functionality out of experimental The share-safe functionality is complete and all configuration options are implemented. The behavior is well discussed on mailing list and in reviews. Let's unmark this as experimental to solve a chichen and egg issue. Differential Revision: https://phab.mercurial-scm.org/D9823

share: rename share-safe warning config Config introduced in previous patch was `share.source-safe-mismatch`. Let's rename the warn as `share.source-safe-mismatch.warn`. While we are here, made sure we have different configs for upgrade and downgrade. Differential Revision: https://phab.mercurial-scm.org/D9786