sshpeer: make "instance" a function

The API is that peer modules must provide an "instance" symbol
that is callable to return a peer.

Making "instance" a function instead of an alias to "sshpeer"
makes it easier to monkeypatch the "sshpeer" type. It will also
make it possible to turn instance() into a factory function of
sorts that returns different types based on connection properties.

Differential Revision: https://phab.mercurial-scm.org/D2024

mercurial/sshpeer.py

@@ -370,4 +370,5 b' class sshpeer(wireproto.wirepeer):'
             self._pipeo.flush()
         self._readerr()
 
-instance = sshpeer
+def instance(ui, path, create):
+    return sshpeer(ui, path, create=create)