Fix more check_output calls in PRESUBMIT.py for python3

Change-Id: I57dc02844db88ec61549c7b2cb4614d88b0979b5
diff --git a/PRESUBMIT.py b/PRESUBMIT.py
index b916f47..fea3ee4 100644
--- a/PRESUBMIT.py
+++ b/PRESUBMIT.py
@@ -32,7 +32,8 @@
     if not path.endswith('.java'):
       continue
     diff = check_output(
-        ['git', 'diff', '--no-prefix', '-U0', branch, '--', path])
+        ['git', 'diff', '--no-prefix', '-U0', branch, '--', path]).decode('utf-8')
+
     proc = Popen(FMT_CMD, stdin=PIPE, stdout=PIPE, stderr=STDOUT)
     (stdout, stderr) = proc.communicate(input=diff)
     if len(stdout) > 0:
@@ -69,7 +70,7 @@
     if not path.endswith('InternalOptions.java'):
       continue
     diff = check_output(
-        ['git', 'diff', '--no-prefix', '-U0', branch, '--', path])
+        ['git', 'diff', '--no-prefix', '-U0', branch, '--', path]).decode('utf-8')
     if 'DETERMINISTIC_DEBUGGING' in diff:
       return [output_api.PresubmitError(diff)]
   return []