Rewrite update-libfuzzer.sh to vendor a specific commit

Also do a shallow and sparse clone to make running the script faster.

update-libfuzzer.sh

@@ -2,20 +2,26 @@
 
 # Usage:
 #
-#     ./update-libfuzzer $commit_hash
-#
-# Where `$commit_hash` is a commit hash from
-# https://github.com/llvm-mirror/llvm-project
+#     $ ./update-libfuzzer.sh
 
 set -ex
 
+# The LLVM commit from which we are vendoring libfuzzer. This must be a commit
+# hash from https://github.com/llvm/llvm-project
+COMMIT=3b5b5c1ec4a3095ab096dd780e84d7ab81f3d7ff
+
 cd "$(dirname $0)"
 project_dir="$(pwd)"
 
 tmp_dir="$(mktemp -d)"
-
-git clone https://github.com/llvm/llvm-project.git "$tmp_dir"
 cd "$tmp_dir"
-git checkout "$1"
+
+git init
+git remote add llvm https://github.com/llvm/llvm-project.git
+git sparse-checkout set compiler-rt/lib/fuzzer
+
+git fetch --depth 1 llvm "$COMMIT" --filter=blob:none
+git checkout "$COMMIT"
+
 rm -rf "$project_dir/libfuzzer/"
 mv "$tmp_dir/compiler-rt/lib/fuzzer/" "$project_dir/libfuzzer/"