Only download compat-table if it's not already downloaded (babel#11081)

* Only download compat-table if it's not already downloaded

* Use git rev-parse

packages/babel-compat-data/scripts/download-compat-table.sh

@@ -2,6 +2,18 @@
 set -e
 
 COMPAT_TABLE_COMMIT=4e9369a699b0e15ba5c21586ce3bdd34299db9c1
+GIT_HEAD=build/compat-table/.git/HEAD
+
+if [ -d "build/compat-table" ]; then
+  cd build/compat-table
+  commit="$(git rev-parse HEAD)"
+  cd ../..
+
+  if [ $commit == $COMPAT_TABLE_COMMIT ]; then
+    exit 0
+  fi
+fi
+
 rm -rf build/compat-table
 mkdir -p build
 git clone --branch=gh-pages --single-branch --shallow-since=2019-11-14 https://github.com/kangax/compat-table.git build/compat-table