R1:99ee9357eff2

Updates for Mercurial's HTTP protocol

Summary:
While testing https://secure.phabricator.com/D21864 I ran into some issues getting mercurial HTTP access working. Using wireshark I confirmed that my local mercurial 6.4 was not including command arguments as HTTP headers but in the querystring.

I didn't dig too deep into understanding when/why this started happening. The protocol documents this in [[ https://repo.mercurial-scm.org/hg/file/tip/mercurial/helptext/internals/wireprotocol.txt | wireprotocol.txt…