"git fetch $there $refspec" that talks over protocol v2 can take
advantage of server-side ref filtering; the code has been extended
so that this mechanism triggers also when fetching with configured
refspec.

* bw/ref-prefix-for-configured-refspec: (38 commits)
  fetch: generate ref-prefixes when using a configured refspec
  refspec: consolidate ref-prefix generation logic
  submodule: convert push_unpushed_submodules to take a struct refspec
  remote: convert check_push_refs to take a struct refspec
  remote: convert match_push_refs to take a struct refspec
  http-push: store refspecs in a struct refspec
  transport: remove transport_verify_remote_names
  send-pack: store refspecs in a struct refspec
  transport: convert transport_push to take a struct refspec
  push: convert to use struct refspec
  push: check for errors earlier
  remote: convert match_explicit_refs to take a struct refspec
  remote: convert get_ref_match to take a struct refspec
  remote: convert query_refspecs to take a struct refspec
  remote: convert apply_refspecs to take a struct refspec
  remote: convert get_stale_heads to take a struct refspec
  fetch: convert prune_refs to take a struct refspec
  fetch: convert get_ref_map to take a struct refspec
  fetch: convert do_fetch to take a struct refspec
  refspec: remove the deprecated functions
  ...