The retry backend does not return the original error, if its execution
is interrupted by canceling the context. Thus, we have to manually
ensure that the invalid data error gets returned.
Additionally, use the retry backend for some of the repository tests, as
this is the configuration which will be used by restic.
TestRepository and its variants always returned no-op cleanup functions.
If they ever do need to do cleanup, using testing.T.Cleanup is easier
than passing these functions around.
As MergeFinalIndex and index uploads can occur concurrently, it is
necessary for MergeFinalIndex to check whether the IDs for an index were
already set before merging it. Otherwise, we'd loose the ID of an index
which is set _after_ uploading it.
