sslutil: handle default CA certificate loading on Windows

See the inline comment for what's going on here.

There is magic built into the "ssl" module that ships with modern
CPython that knows how to load the system CA certificates on
Windows. Since we're not shipping a CA bundle with Mercurial,
if we're running on legacy CPython there's nothing we can do
to load CAs on Windows, so it makes sense to print a warning.

I don't anticipate many people will see this warning because
the official (presumed popular) Mercurial distributions on
Windows bundle Python and should be distributing a modern Python
capable of loading system CA certs.

mercurial/sslutil.py

@@ -447,6 +447,19 b' def _defaultcacerts(ui):'
     except ImportError:
         pass
 
+    # On Windows, only the modern ssl module is capable of loading the system
+    # CA certificates. If we're not capable of doing that, emit a warning
+    # because we'll get a certificate verification error later and the lack
+    # of loaded CA certificates will be the reason why.
+    # Assertion: this code is only called if certificates are being verified.
+    if os.name == 'nt':
+        if not _canloaddefaultcerts:
+            ui.warn(_('(unable to load Windows CA certificates; see '
+                      'https://mercurial-scm.org/wiki/SecureConnections for '
+                      'how to configure Mercurial to avoid this message)\n'))
+
+        return None
+
     # Apple's OpenSSL has patches that allow a specially constructed certificate
     # to load the system CA store. If we're running on Apple Python, use this
     # trick.

tests/test-https.t

@@ -60,6 +60,13 b' we are able to load CA certs.'
   [255]
 #endif
 
+#if no-sslcontext windows
+  $ hg clone https://localhost:$HGPORT/ copy-pull
+  (unable to load Windows CA certificates; see https://mercurial-scm.org/wiki/SecureConnections for how to configure Mercurial to avoid this message)
+  abort: error: *certificate verify failed* (glob)
+  [255]
+#endif
+
 #if defaultcacertsloaded
   $ hg clone https://localhost:$HGPORT/ copy-pull
   abort: error: *certificate verify failed* (glob)