hidden: add support to explicitly access hidden changesets via HTTP

We now detect hidden access request and validate them while handling HTTP
wireprotocol command. This changeset only introduces server support. Client
support is added in a subsequent changeset.

The mechanism we use for the HTTP wireprotocol is the same as the one used to
lift filtering for hgweb. The use of the global `access-hidden` parameter in
the http url is a simple and effective approach that

* works around the lack of global parameters in wire protocol v1,
* reuses the mechanism used for hgweb (that does not use the wireproto), and
* can be safely ignored by older client (fitting the best effort contract).

The feature is still experimental, so we can easily update the implementation
details in the future. However, landing a first implementation now will help
test the feature in the wild and iterate on alternative implementations if
deemed necessary.

mercurial/wireprotoserver.py

@@ -317,7 +317,8 b' def _callhttp(repo, req, res, proto, cmd'
 
     proto.checkperm(wireprotov1server.commands[cmd].permission)
 
-    rsp = wireprotov1server.dispatch(repo, proto, cmd)
+    accesshidden = hgwebcommon.hashiddenaccess(repo, req)
+    rsp = wireprotov1server.dispatch(repo, proto, cmd, accesshidden)
 
     if isinstance(rsp, bytes):
         setresponse(HTTP_OK, HGTYPE, bodybytes=rsp)