Fix incorrect use of extend() in git_sync_cl_chain.py

Change-Id: If65851d81634e08f3e7b25bb29e62b88d584b6d1
diff --git a/tools/git_sync_cl_chain.py b/tools/git_sync_cl_chain.py
index f3a0728..857aab8 100755
--- a/tools/git_sync_cl_chain.py
+++ b/tools/git_sync_cl_chain.py
@@ -142,7 +142,9 @@
 
 def delete_branches(branches):
   assert len(branches) > 0
-  utils.RunCmd(['git', 'branch', '-D'].extend(branches), quiet=True)
+  cmd = ['git', 'branch', '-D']
+  cmd.extend(branches)
+  utils.RunCmd(cmd, quiet=True)
 
 def get_branch_with_name(name, branches):
   for branch in branches: