ref: add update script that gets the latest commit.

update/ref

@@ -0,0 +1,15 @@
+# /usr/bin/env bash
+set -euo pipefail
+IFS=$'\n\t'
+set -vx
+
+tree_sitter='https://github.com/tree-sitter/tree-sitter'
+langs=$(cat $PWD/ref | cut -d ':' -f 1)
+
+for lang in $langs
+do
+  ref=$(git ls-remote "$tree_sitter-$lang" refs/heads/master | sed -E 's/^([0-9a-fA-F]+)\s.*$/\1/')
+  echo "$lang:$ref" >> ref.new
+done
+
+mv ref.new ref