test-rebase-conflicts: add a test case about turning obsstore on and off Turning obsstore and allowunstable on, rebase will skip the "can't remove original changesets with unrebased descendants" check. Then rebase could be interrupted (merge conflict), and the user has a chance to turn off obsstore. If rebase continues, the current code may strip irrelevant commits (in the test case added, "C" got stripped unexpectedly). The test case reproduces issue5606. It will be fixed by the "multidest" rebase refactoring being reviewed. The test case itself is relatively separate from the rebase refactoring, therefore sent separately hoping to reduce the number of patches of the main rebase series.