Fix script for getting git HEAD revision

Change-Id: Ia1b2a0cf8220b212ba8fa158614f5d46454cf9e0
diff --git a/tools/git_utils.py b/tools/git_utils.py
index 1e3e7c5..1ce0a0a 100644
--- a/tools/git_utils.py
+++ b/tools/git_utils.py
@@ -16,4 +16,4 @@
   cmd = ['git', 'rev-parse', revision_from]
   utils.PrintCmd(cmd)
   with utils.ChangedWorkingDirectory(checkout_dir):
-    return subprocess.check_output(cmd).strip()
+    return subprocess.check_output(cmd).strip().decode('utf-8')