feat: add plugin trigger for checking if an app has code

This will allow us to short-circuit build processes that would otherwise be triggered due to various commands in Dokku.

plugins/git/internal-functions

@@ -405,6 +405,22 @@ fn-git-fetch() {
   fn-git-cmd "$APP_ROOT" config --add core.bare true
 }
 
+fn-git-has-code() {
+  declare desc="checks if there is code at a specific branch"
+  declare APP="$1" DOKKU_DEPLOY_BRANCH="$2"
+
+  if [[ -z "$DOKKU_DEPLOY_BRANCH" ]]; then
+    DOKKU_DEPLOY_BRANCH="$(fn-git-deploy-branch "$APP")"
+  fi
+
+  local HEAD_FILE="$DOKKU_ROOT/$APP/refs/heads/$DOKKU_DEPLOY_BRANCH"
+  if [[ -f "$HEAD_FILE" ]]; then
+    return 0
+  fi
+
+  return 1
+}
+
 fn-git-last-updated-at() {
   declare desc="retrieve the deploy branch for a given application"
   declare APP="$1"