Use --style file instead of --style LLVM

LLVM includes .clang-format files which must be respected.
diff --git a/utils/arcanist/clang-format.sh b/utils/arcanist/clang-format.sh
index 4fa2dd9..fba3e2a 100755
--- a/utils/arcanist/clang-format.sh
+++ b/utils/arcanist/clang-format.sh
@@ -41,7 +41,7 @@
 if git rev-parse --git-dir >/dev/null; then
   arc_base_commit=$(arc which --show-base)
   # An alternative is to use git-clang-format.
-  >&2 git-clang-format --quiet --force --style LLVM "${arc_base_commit}"
+  >&2 git-clang-format --quiet --force --style file "${arc_base_commit}"
 else
   >&2 echo "repo is expected to be a git directory"
 fi