gclient: run cipd ensure in paranoid mode.

Bug: 860251
Change-Id: I0acaafcfe1d214d102b6a17b29b76aadcc1542c2
Reviewed-on: https://chromium-review.googlesource.com/1132345
Commit-Queue: John Budorick <jbudorick@chromium.org>
Reviewed-by: Vadim Shtayura <vadimsh@chromium.org>
Reviewed-by: Robbie Iannucci <iannucci@chromium.org>

gclient_scm.py

@@ -1343,6 +1343,7 @@ class CipdRoot(object):
       ensure_file = None
       with tempfile.NamedTemporaryFile(
           suffix='.ensure', delete=False) as ensure_file:
+        ensure_file.write('$ParanoidMode CheckPresence\n\n')
         for subdir, packages in sorted(self._packages_by_subdir.iteritems()):
           ensure_file.write('@Subdir %s\n' % subdir)
           for package in sorted(packages, key=lambda p: p.name):

tests/gclient_smoketest.py

@@ -1801,6 +1801,8 @@ class GClientSmokeCipd(GClientSmokeBase):
       contents = f.read()
 
     self.assertEqual([
+        '$ParanoidMode CheckPresence',
+        '',
         '@Subdir src/another_cipd_dep',
         'package1 1.1-cr0',
         'package2 1.13',