Gitea review and execution mismatch through duplicate tree entries

Gitea accepted pushed Git trees containing two entries with the same name, which Git's own consistency checks reject. Gitea's web views resolved such a path to the first entry, while `git checkout`, Gitea Actions, and release archives use the last. A contributor could open a pull request whose diff and file views show benign content while CI and checkouts at the same commit use different, attacker-controlled content. Incoming objects are now checked for consistency; objects already stored in existing repositories are not rescanned.