dummyssh: make sure we don't inherit files descriptor to the children

This is one of the difference between python2 and python3 and could have been a
reason why test hang with python2 + chg. This does not seems to help the
hanging issue at all…

However, now that this is written lets reduce the difference between python2
and python3.

Differential Revision: https://phab.mercurial-scm.org/D11225

tests/dummyssh

@@ -1,35 +1,35 b''
 #!/usr/bin/env python3
 
 from __future__ import absolute_import
 
 import os
 import shlex
 import subprocess
 import sys
 
 os.chdir(os.getenv('TESTTMP'))
 
 if sys.argv[1] != "user@dummy":
     sys.exit(-1)
 
 os.environ["SSH_CLIENT"] = "%s 1 2" % os.environ.get('LOCALIP', '127.0.0.1')
 
 log = open("dummylog", "ab")
 log.write(b"Got arguments")
 for i, arg in enumerate(sys.argv[1:]):
     log.write(b" %d:%s" % (i + 1, arg.encode('latin1')))
 log.write(b"\n")
 log.close()
 hgcmd = sys.argv[2]
 if os.name == 'nt':
     # hack to make simple unix single quote quoting work on windows
     hgcmd = hgcmd.replace("'", '"')
     cmds = shlex.split(hgcmd)
     if cmds[0].endswith('.py'):
         python_exe = os.environ['PYTHON']
         cmds.insert(0, python_exe)
     hgcmd = shlex.join(cmds)
     # shlex generate windows incompatible string...
     hgcmd = hgcmd.replace("'", '"')
-r = subprocess.call(hgcmd, shell=True)
+r = subprocess.call(hgcmd, shell=True, close_fds=True)
 sys.exit(bool(r))