gsutil_util: replace `du` with `stat`

`gsutil du PATH` no longer returns an error even if the file does not
exist. Use `gsutil stat` instead.

BUG=chromium:434029
TEST=Manually run the commands using different gsutil versions

Change-Id: I7af64735a2b6dae67fb2b724792c1734da3f3511
Reviewed-on: https://chromium-review.googlesource.com/230305
Tested-by: Yu-Ju Hong <yjhong@chromium.org>
Reviewed-by: Chris Sosa <sosa@chromium.org>
Commit-Queue: Yu-Ju Hong <yjhong@chromium.org>
diff --git a/gsutil_util_unittest.py b/gsutil_util_unittest.py
index 6ac85cd..3071825 100755
--- a/gsutil_util_unittest.py
+++ b/gsutil_util_unittest.py
@@ -109,7 +109,7 @@
     self.mox.StubOutWithMock(gsutil_util, 'GSUtilRun')
 
     # GSUtil cat gs://archive_url_prefix/UPLOADED.
-    gsutil_util.GSUtilRun(mox.StrContains('gsutil du %s/%s' %
+    gsutil_util.GSUtilRun(mox.StrContains('gsutil stat %s/%s' %
                                           (archive_url, pattern)),
                           mox.IgnoreArg()).AndReturn(
                               '%s\n%s' % (mock_data, name))