sslutil: abort if peer certificate is not verified for secure use

Before this patch, "sslutil.validator" may returns successfully, even
if peer certificate is not verified because there is no information in
"[hostfingerprints]" and "[web] cacerts".

To prevent from sending authentication credential to untrustable SMTP
server, validation should be aborted if peer certificate is not
verified.

This patch introduces "strict" optional argument, and
"sslutil.validator" will abort if it is True and peer certificate is
not verified.

mercurial/sslutil.py

@@ -99,7 +99,7 b' class validator(object):'
         self.ui = ui
         self.host = host
 
-    def __call__(self, sock):
+    def __call__(self, sock, strict=False):
         host = self.host
         cacerts = self.ui.config('web', 'cacerts')
         hostfingerprint = self.ui.config('hostfingerprints', host)
@@ -107,6 +107,9 b' class validator(object):'
             if hostfingerprint:
                 raise util.Abort(_("host fingerprint for %s can't be "
                                    "verified (Python too old)") % host)
+            if strict:
+                raise util.Abort(_("certificate for %s can't be verified "
+                                   "(Python too old)") % host)
             if self.ui.configbool('ui', 'reportoldssl', True):
                 self.ui.warn(_("warning: certificate for %s can't be verified "
                                "(Python too old)\n") % host)
@@ -142,6 +145,11 b' class validator(object):'
                                         '--insecure to connect insecurely') %
                                       nicefingerprint)
             self.ui.debug('%s certificate successfully verified\n' % host)
+        elif strict:
+            raise util.Abort(_('%s certificate with fingerprint %s not '
+                               'verified') % (host, nicefingerprint),
+                             hint=_('check hostfingerprints or web.cacerts '
+                                     'config setting'))
         else:
             self.ui.warn(_('warning: %s certificate with fingerprint %s not '
                            'verified (check hostfingerprints or web.cacerts '