filelog: record what's using attributes filelog and the file storage interface have some attributes that ideally shouldn't be there. This commit annotates some of those attributes with their users so we know where to look when it comes time to removing them. This exercise exposed a theme: many attributes are used by LFS, repo upgrade, verify, and special repo types (like bundlerepo). That points to missing abstractions on file storage to facilitate these special needs. Differential Revision: https://phab.mercurial-scm.org/D4661