The beginning of the next-gen transfer protocol.

* bw/protocol-v2: (35 commits)
  remote-curl: don't request v2 when pushing
  remote-curl: implement stateless-connect command
  http: eliminate "# service" line when using protocol v2
  http: don't always add Git-Protocol header
  http: allow providing extra headers for http requests
  remote-curl: store the protocol version the server responded with
  remote-curl: create copy of the service name
  pkt-line: add packet_buf_write_len function
  transport-helper: introduce stateless-connect
  transport-helper: refactor process_connect_service
  transport-helper: remove name parameter
  connect: don't request v2 when pushing
  connect: refactor git_connect to only get the protocol version once
  fetch-pack: support shallow requests
  fetch-pack: perform a fetch using v2
  upload-pack: introduce fetch server command
  push: pass ref prefixes when pushing
  fetch: pass ref prefixes when fetching
  ls-remote: pass ref prefixes when requesting a remote's refs
  transport: convert transport_get_remote_refs to take a list of ref prefixes
  ...

Fix a typo in an error message.

Also, this line was introduced in 3145ea9 ("upload-pack: introduce
fetch server command", 2018-03-15), which did not contain a test for the
case which causes this error to be printed, so introduce a test.

Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

The upload-pack code paths never call git_config() with
upload_pack_config() when protocol v2 is used, causing options like
uploadpack.packobjectshook to not take effect. Ensure that this function
is called.

Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

The fetch-pack/upload-pack protocol v2 was developed independently of
the filter parameter (used in partial fetches), thus it did not include
support for it. Add support for the filter parameter.

Like in the legacy protocol, the server advertises and supports "filter"
only if uploadpack.allowfilter is configured.

Like in the legacy protocol, the client continues with a warning if
"--filter" is specified, but the server does not advertise it.

Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>