py3: fix hanging read of SSH subprocesses

Summary:
Previously, reading from SSH subprocesses did’t return on Python 3.  bufsize=0
is default on Python 2, but not Python 3.

This is a backport of: 0fc8020ebe

Reviewed By: krallin

Differential Revision: D24652418

fbshipit-source-id: 1140c76b6f711bfe1726108bd4fe6948e6ee41a0

eden/scm/edenscm/hgext/hggit/_ssh.py

@@ -30,6 +30,7 @@ def generate_ssh_vendor(ui):
             ui.debug("calling ssh: %s\n" % cmd)
             proc = subprocess.Popen(
                 util.quotecommand(cmd),
+                bufsize=0,
                 shell=True,
                 stdin=subprocess.PIPE,
                 stdout=subprocess.PIPE,