Enable the clang-format pre-submit hook

Bug: 36699008
Test: Try to upload a patch with formatting errors.
Change-Id: I4af7d142ccbbaf39eb236547487c59c5ff8f2cd6
diff --git a/PREUPLOAD.cfg b/PREUPLOAD.cfg
new file mode 100644
index 0000000..213c93a
--- /dev/null
+++ b/PREUPLOAD.cfg
@@ -0,0 +1,5 @@
+[Options]
+ignore_merged_commits = true
+
+[Builtin Hooks]
+clang_format = true