Merge branch 'jh/fsck-promisors'

In preparation for implementing narrow/partial clone, the machinery for checking object connectivity used by gc and fsck has been taught that a missing object is OK when it is referenced by a packfile specially marked as coming from trusted repository that promises to make them available on-demand and lazily. * jh/fsck-promisors: gc: do not repack promisor packfiles rev-list: support termination at promisor objects sha1_file: support lazily fetching missing objects introduce fetch-object: fetch one promisor object index-pack: refactor writing of .keep files fsck: support promisor objects as CLI argument fsck: support referenced promisor objects fsck: support refs pointing to promisor objects fsck: introduce partialclone extension extension.partialclone: introduce partial clone extension