Fixes#12645. Detailed analysis in [this comment](https://codeberg.org/forgejo/forgejo/issues/12645#issuecomment-15939122). New test case is verified to hit the bug -- the previous case just narrowly missed the problem because it ended up with an empty repository index.
## Checklist
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### Tests for Go changes
- I added test coverage for Go changes...
- [ ] in their respective `*_test.go` for unit tests.
- [x] in the `tests/integration` directory if it involves interactions with a live Forgejo server.
- I ran...
- [x] `make pr-go` before pushing
### Documentation
- [ ] I created a pull request [to the documentation](https://codeberg.org/forgejo/docs) to explain to Forgejo users how to use this change.
- [x] I did not document these changes and I do not expect someone else to do it.
### Release notes
- [x] This change will be noticed by a Forgejo user or admin (feature, bug fix, performance, etc.). I suggest to include a release note for this change.
- [ ] This change is not visible to a Forgejo user or admin (refactor, dependency upgrade, etc.). I think there is no need to add a release note for this change.
Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/12764
Reviewed-by: Andreas Ahlenstorf <aahlenst@noreply.codeberg.org>
