"auto filter" logic for large-object promisor remote.
* cc/lop-filter-auto:
fetch-pack: wire up and enable auto filter logic
promisor-remote: change promisor_remote_reply()'s signature
promisor-remote: keep advertised filters in memory
list-objects-filter-options: support 'auto' mode for --filter
doc: fetch: document `--filter=<filter-spec>` option
fetch: make filter_options local to cmd_fetch()
clone: make filter_options local to cmd_clone()
promisor-remote: allow a client to store fields
promisor-remote: refactor initialising field lists