[ci] Print git revision of elasticsearch to ci log (elastic/elasticsearch#4782)

The output `Checked out Elasticsearch revision: ...` shows which revision of elasticsearch was included in the CI build. This makes it easier to diagnose and resolve x-pack build failures.

Original commit: elastic/x-pack-elasticsearch@8c39aafc2d

dev-tools/ci

@@ -96,6 +96,7 @@ if [ -z ${USE_EXISTING_ES:+x} ]; then
     fi
     echo "Checking out Elasticsearch '$BRANCH' branch..."
     git clone -b $BRANCH https://github.com/elastic/elasticsearch.git --depth=1
+    printf "Checked out Elasticsearch revision: %s\n" "$(git -C elasticsearch rev-parse HEAD)"
 else
     if [ -d "./elasticsearch" ]; then
         echo "Using existing 'elasticsearch' checkout"