Add introducing-commit references to the package commit template #29

The package commit template does not cover references to the commit that introduced a bug. The current template restates the attribution rule. The conditional introducing-commit reference is proposed in cozystack/cozystack#4859.

After that guidance lands, update both the template and the generated message: when a bump fixes a defect introduced by a known commit, include its hash and subject before attribution and sign-off.

Fixes: <at-least-12-character-sha> ("full introducing commit subject")

Do not invent an introducing commit for an ordinary upgrade. Keep issue references separate: use a closing reference only for a complete fix, and qualify references to another repository.