Locally record the manifest before uploading it or any bundles,
and read it on the next push. Any bundles from the push that are
not included in the currently being pushed manifest will get added
to the outManifest, and so eventually get deleted.
This deals with an interrupted push that is not resumed and instead
something else is pushed. And it deals with a push race that overwrites
the manifest.
Of course, this can't help if one of those situations is followed by
the local repo being deleted. But that's equivilant to doing a git-annex
copy of a new annexed file to a special remote and then deleting the
special repo w/o pushing. In either case the special remote ends up with
a object in it that git-annex doesn't know about.
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