dir: Fix another deploy error code path

This is another case where commit 4beaa990c seems to have mistakenly
turned an error code path into one where the deploy appears successful
to the caller of flatpak_dir_deploy() but the commit doesn't actually
get deployed.
This commit is contained in:
Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f