wireprotov2: allow multiple fields to follow revision maps The *data wire protocol commands emit a series of CBOR values. Because revision/delta data may be large, their data is emitted outside the map as a top-level bytestring value. Before this commit, we'd emit a single optional bytestring value after the revision descriptor map. This got the job done. But it was limiting in that we could only send a single field. And, it required the consumer to know that the presence of a key in the map implied the existence of a following bytestring value. This commit changes the encoding strategy so top-level bytestring values in the stream are explicitly denoted in a "fieldsfollowing" key. This key contains an array defining what fields that follow and the expected size of each field. By defining things this way, we can easily send N bytestring values without any ambiguity about their order. In addition, clients only need to know how to parse ``fieldsfollowing`` to know if extra values are present. Because this breaks backwards compatibility, we've bumped the version number of the wire protocol version 2 API endpoint. Differential Revision: https://phab.mercurial-scm.org/D4620

--- a/tests/test-ssh.t	Mon Sep 17 11:54:00 2018 -0700
+++ b/tests/test-ssh.t	Thu Sep 20 12:57:23 2018 -0700
@@ -488,13 +488,13 @@
   $ hg pull --debug ssh://user@dummy/remote --config devel.debug.peer-request=yes
   pulling from ssh://user@dummy/remote
   running .* ".*/dummyssh" ['"]user@dummy['"] ('|")hg -R remote serve --stdio('|") (re)
-  sending upgrade request: * proto=exp-ssh-v2-0001 (glob) (sshv2 !)
+  sending upgrade request: * proto=exp-ssh-v2-0002 (glob) (sshv2 !)
   devel-peer-request: hello+between
   devel-peer-request:   pairs: 81 bytes
   sending hello command
   sending between command
   remote: 427 (sshv1 !)
-  protocol upgraded to exp-ssh-v2-0001 (sshv2 !)
+  protocol upgraded to exp-ssh-v2-0002 (sshv2 !)
   remote: capabilities: batch branchmap $USUAL_BUNDLE2_CAPS$ changegroupsubset getbundle known lookup protocaps pushkey streamreqs=generaldelta,revlogv1 unbundle=HG10GZ,HG10BZ,HG10UN unbundlehash
   remote: 1 (sshv1 !)
   devel-peer-request: protocaps