wireproto: port heads command to wire protocol v2 After much thought and consideration, wire protocol version 2's commands will be defined in different functions from the existing commands. This will make it easier to implement these commands because it won't require shoehorning things like response formatting and argument declaration into the same APIs. For example, wire protocol version 1 requires that commands declare a fixed and ordered list of argument names. It isn't really possible to insert new arguments or have optional arguments without breaking backwards compatibility. Wire protocol version 2, however, uses CBOR maps for passing arguments. So arguments a) can be optional b) can be added without BC c) can be strongly typed. This commit starts our trek towards reimplementing the wire protocol for version 2 with the heads command. It is pretty similar to the existing heads command. One added feature is it can be told to operate on only public phase changesets. This is useful for making discovery faster when a repo has tens of thousands of draft phase heads (such as Mozilla's "try" repository). The HTTPv2 server-side protocol has had its `getargs()` implementation updated to reflect that arguments are a map and not a list. Differential Revision: https://phab.mercurial-scm.org/D3179

--- a/mercurial/wireprototypes.py	Wed Mar 28 10:52:40 2018 -0700
+++ b/mercurial/wireprototypes.py	Wed Mar 28 14:55:13 2018 -0700
@@ -97,6 +97,11 @@
     def __init__(self, gen=None):
         self.gen = gen
 
+class cborresponse(object):
+    """Encode the response value as CBOR."""
+    def __init__(self, v):
+        self.value = v
+
 class baseprotocolhandler(zi.Interface):
     """Abstract base class for wire protocol handlers.
 
@@ -115,7 +120,10 @@
     def getargs(args):
         """return the value for arguments in <args>
 
-        returns a list of values (same order as <args>)"""
+        For version 1 transports, returns a list of values in the same
+        order they appear in ``args``. For version 2 transports, returns
+        a dict mapping argument name to value.
+        """
 
     def getprotocaps():
         """Returns the list of protocol-level capabilities of client