localrepo: support marking repos as having shallow file storage Various operations against repositories need to know if repository storage is full or partial. For example, a checkout (including possibly a widening of a sparse checkout), needs to know if it can assume all file revisions are available or whether to look for missing revisions first. This commit lays the plumbing for doing that. We define a repo creation option that indicates that shallow file storage is desired. The SQLite store uses this creation option to add an extra repo requirement indicating file storage is shallow. A new repository feature has been added to indicate that file storage is shallow. The SQLite store adds this feature when the shallow file store requirement is present. Code can now look at repo.features to determine if repo file storage may be shallow and take additional actions if so. While we're here, we also teach the SQLite store to handle the narrow repo requirement, which gets added when making narrow clones. Differential Revision: https://phab.mercurial-scm.org/D5166