Merge branch 'jn/promote-proto2-to-default'

Merge branch 'jn/promote-proto2-to-default'

The transport protocol version 2 becomes the default one.

* jn/promote-proto2-to-default:
  fetch: default to protocol version 2
  protocol test: let protocol.version override GIT_TEST_PROTOCOL_VERSION
  test: request GIT_TEST_PROTOCOL_VERSION=0 when appropriate
  config doc: protocol.version is not experimental
  fetch test: use more robust test for filtered objects