ssl: load CA certificates from system's store by default on Python 2.7.9

This will make it easy to manage in-house CA certificates, which are often
used in corporate environment and installed into the Windows' certs store.

Unlike Apple python, the dummycert trick isn't necessary on Python 2.7.9.
The default web.cacerts will be set as follows:

  environment    web.cacerts  behavior
  -------------  -----------  -----------------------------------------
  Apple Python   dummycert    fall back to system's store
  Python 2.7.8   '!'          never use CA certs (show warning instead)
  Python 2.7.9+  None         load CA certs from system's store

mercurial/sslutil.py

@@ -10,12 +10,16 b' import os, sys'
 
 from mercurial import util
 from mercurial.i18n import _
+
+_canloaddefaultcerts = False
 try:
     # avoid using deprecated/broken FakeSocket in python 2.6
     import ssl
     CERT_REQUIRED = ssl.CERT_REQUIRED
     try:
         ssl_context = ssl.SSLContext
+        _canloaddefaultcerts = util.safehasattr(ssl_context,
+                                                'load_default_certs')
 
         def ssl_wrap_socket(sock, keyfile, certfile, cert_reqs=ssl.CERT_NONE,
                             ca_certs=None, serverhostname=None):
@@ -35,6 +39,8 b' try:'
             sslcontext.verify_mode = cert_reqs
             if ca_certs is not None:
                 sslcontext.load_verify_locations(cafile=ca_certs)
+            elif _canloaddefaultcerts:
+                sslcontext.load_default_certs()
 
             sslsocket = sslcontext.wrap_socket(sock,
                                                server_hostname=serverhostname)
@@ -130,10 +136,13 b' def _plainapplepython():'
             exe.startswith('/system/library/frameworks/python.framework/'))
 
 def _defaultcacerts():
+    """return path to CA certificates; None for system's store; ! to disable"""
     if _plainapplepython():
         dummycert = os.path.join(os.path.dirname(__file__), 'dummycert.pem')
         if os.path.exists(dummycert):
             return dummycert
+    if _canloaddefaultcerts:
+        return None
     return '!'
 
 def sslkwargs(ui, host):