Fix abort-on-error behaviour of transactions

[smcv]

• repo: Factor out _ostree_repo_auto_transaction_new()

  This will allow the direct allocation in
  ostree_repo_prepare_transaction() to be replaced with a call to this
  function, avoiding breaking encapsulation.

• repo: Correctly initialize refcount of temporary transaction

  Previously, the reference count was left uninitialized as a result of
  bypassing the constructor, and the intended abort-on-error usually
  wouldn't have happened.

  Fixes: 8a9737a "repo/private: move OstreeRepoAutoTransaction to a boxed type"
  Resolves: OstreeRepoAutoTransaction used with uninitialized refcount in ostree_repo_prepare_transaction() #2592

[openshift-ci]

Hi @smcv. Thanks for your PR.

I'm waiting for a ostreedev member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

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[smcv]

I think this is ready for review, but I'm marking it as a draft while I see whether it fixes the CI for flatpak/flatpak#4326.

[dbnicholson]

/ok-to-test

[lucab]

/approve
/ok-to-test

[smcv]

I think this is right, but I won't know whether it fixed my failing CI for at least another hour (running the Flatpak test-suite under valgrind is slow). https://github.com/flatpak/flatpak/runs/6240039656?check_suite_focus=true

[smcv]

The Flatpak test-suite has finally passed under valgrind, and yes, a backport of these commits to 2022.2 does resolve the test failure.