wireprotoserver: handle SSH protocol version 2 upgrade requests This commit teaches the SSH server to recognize the "upgrade" request line that clients send when they wish to switch the channel to version 2 of the SSH protocol. Servers don't honor upgrade requests unless an experimental config option is set. Since the built-in server now supports upgrade requests, our test server to test the handshake has been deleted. Existing tests use the built-in server and their output doesn't change. The upgrade is handled in our state machine. The end result is a bit wonky, as the server transitions back to version 1 state immediately after upgrading. But this will change as soon as version 2 has an actual protocol that differs from version 1. Tests demonstrating that the new server is a bit more strict about the upgrade handshake have been added. Differential Revision: https://phab.mercurial-scm.org/D2204

--- a/mercurial/configitems.py	Thu Feb 08 15:09:59 2018 -0800
+++ b/mercurial/configitems.py	Mon Feb 12 16:33:54 2018 -0800
@@ -556,6 +556,9 @@
 coreconfigitem('experimental', 'single-head-per-branch',
     default=False,
 )
+coreconfigitem('experimental', 'sshserver.support-v2',
+    default=False,
+)
 coreconfigitem('experimental', 'spacemovesdown',
     default=False,
 )