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 wireprotocol.txt.

Command arguments can be sent multiple ways. The simplest is part of the URL query string using `x-www-form-urlencoded encoding (see Python's urllib.urlencode()`. However, many servers impose length limitations on the URL. So this mechanism is typically only used if the server doesn't support other mechanisms.

Based on that either the mercurial on the server is really old (it's 6.1.1 tho) or maybe some other parsing/info passing in Phab's handling of the wire protocol is causing the client to downgrade the wire protocol support.

Cherry-picked from:

https://secure.phabricator.com/D21867

https://secure.phabricator.com/rP0b6e758978a9691bd5ad25db4aa4c4301640a9a9

Test Plan: Host mercurial repo using HTTP, test push/pull.

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: tobiaswiese, Matthew, Cigaryno

Differential Revision: https://we.phorge.it/D25471