Improve clang-format vim integration.

The previous line-by-line replacement causes vim to take a long time if
the foldmethod is set to 'syntax'. This should significantly improve
performance in that case.

git-svn-id: svn://svn.chromium.org/llvm-project/cfe/trunk/tools/clang-format@186660 0b72dbe1-c17f-4bc7-b9db-2b4152be0356
diff --git a/clang-format.py b/clang-format.py
index 60e2e55..29be72e 100644
--- a/clang-format.py
+++ b/clang-format.py
@@ -71,8 +71,8 @@
   output = json.loads(lines[0])
   lines = lines[1:]
   if '\n'.join(lines) != text:
-    for i in range(min(len(buf), len(lines))):
-      buf[i] = lines[i]
+    common_length = min(len(buf), len(lines))
+    buf[:common_length] = lines[:common_length]
     for line in lines[len(buf):]:
       buf.append(line)
     del buf[len(lines):]