build-update-repo: Don't try to generate deltas of unknown refs
build-update-repo: Don't try to generate deltas of unknown refs

Spotted this by code inspection. Ignoring such refs was clearly the
intended behavior based on the comment.

(cherry picked from commit 17fbe516c4251c08a648f5e938af3af78fbd4ab4)

flatpak/flatpak,flatpak/flatpak,flatpak/flatpak,flatpak/flatpak,flatpak/flatpak