Made gsutil.py truly transparent

The previous version was modifying gsutil output by adding a
stacktrace upon non-zero return code, which broke existing code.

BUG=400871
R=hinoka@chromium.org

Review URL: https://codereview.chromium.org/822093002

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@293501 0039d316-1c4b-4281-b951-d872f2087c98

gsutil.py

@@ -125,7 +125,7 @@ def run_gsutil(force_version, fallback, target, args):
   else:
     gsutil_bin = fallback
   cmd = [sys.executable, gsutil_bin] + args
-  call(cmd)
+  os.execv(cmd[0], cmd)
 
 
 def parse_args():