Even for a branch, we should be able to find proper hash. The branch is put in as: git://git.fedorahosted.org/git/spin-kickstarts.git?#origin/f24 (otherwise koji will not be able to check it out).
git://git.fedorahosted.org/git/spin-kickstarts.git?#origin/f24
Fix in PR #185.
This issue has been migrated to Fedora Forge: https://forge.fedoraproject.org/pungi/pungi/issues/183
Please continue any further discussion there.