Use decode('utf-8') for all subprocess calls

Bug: 221778256
Change-Id: Ib2029e5061193a62f3712baba99a395f4340ba86
diff --git a/tools/internal_test.py b/tools/internal_test.py
index 1ec2a19..0d591ad 100755
--- a/tools/internal_test.py
+++ b/tools/internal_test.py
@@ -166,7 +166,7 @@
 
 def ensure_git_clean():
   # Ensure clean git repo.
-  diff = str(subprocess.check_output(['git', 'diff']))
+  diff = subprocess.check_output(['git', 'diff']).decode('utf-8')
   if len(diff) > 0:
     log('Local modifications to the git repo, exiting')
     sys.exit(1)