builtin/index-pack: fix deferred fsck outside repos

When asked to perform object consistency checks via the `--fsck-objects` flag we verify that each object part of the pack is valid. In general, this check can even be performed outside of a Git repository: we don't need an initialized object database as we simply read the object from the packfile directly. But there's one exception: a subset of the object checks may be deferred to a later point in time. For now, this only concerns ".gitmodules" and ".gitattributes" files: whenever we see a tree referencing these files we queue them for a deferred check. This is done because we need to do some extra checks for those files to ensure that they are well-formed, and these checks need to be done regardless of whether the corresponding blobs are part of the packfile or not. This works inside a repository, but unfortunately the logic leads to a segfault when running outside of one. This is because we eventually call `odb_read_object()`, which will crash because the object database has not been initialized. There's multiple options here: - We could in theory create a purely in-memory database with only a packfile store that contains the single packfile. We don't really have the infrastructure for this yet though, and it would end up being quite hacky. - We could refuse to perform consistency checks outside of a repository. But most of the checks work alright, so this would be a regression. - We can skip the finalizing consistency checks when running outside of a repository. This is not as invasive as skipping all checks, but it's not great to randomly skip a subset of tests, either. None of these options really feel perfect. The first one would be the obvious choice if easily possible. There's another option though: instead of skipping the final object checks, we can die if there are any queued object checks. With this change we now die exactly if and only if we would have previously segfaulted. Like this we ensure that objects that _may_ fail the consistency checks won't be silently skipped, and at the same time we give users a much better error message. Refactor the code accordingly and add a test that would have triggered the segfault. Note that we also move down the logic to add the packfile to the store. There is no point doing this any earlier than right before we execute `fsck_finish()`, and it ensures that the logic to set up and perform the consistency check is self-contained. Signed-off-by: Patrick Steinhardt <ps@pks.im> Signed-off-by: Junio C Hamano <gitster@pobox.com>