bundle: make applybundle() delegate v1 bundles to applybundle1()

bundle: transpose transaction scope with bundle type switch This moves the transaction with-statements outside of the per-bundle-version switches, so the next patch will be a little simpler.

unbundle: move BundleUnknownFeatureError exception handling out This just moves the BundleUnknownFeatureError exception handling one level up so we collect the bundle2.applybundle{,1}() calls together. applybundle1() will never throw the exception, so it should have no functional consequence.

bundle: make applybundle1() return a bundleoperation See previous commit for motivation. It already lets us share a little bit more code in commands.py.

bundle: add a applybundle1() method This is one step towards removing a bunch of "if isinstance(gen, unbundle20)" by treating bundle1 and bundle2 more similarly. The name may sounds ironic for a method in the bundle2 module, but I didn't think it was worth it yet to create a new 'bundle' module that depends on the 'bundle2' module. Besides, we'll inline the method again later.

bundle: extract _processchangegroup() method The new method applies the changegroup and fills in op.records, sharing a little bit of code between the two callers. We'll add another caller soon.

bundle: make combinechangegroupresults() take a bundleoperation Both callers have a bundleoperation. Passing it in lets us share a bit more code.

bundle: move combineresults() from changegroup to bundle2 The results only need to be combined if they come from a bundle2. More importantly, we'll change its argument to a bundleoperation soon, and then it definitely will no longer belong in changegroup.py.

bundle: remove 'op' argument from applybundle() No callers pass in an operation.

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.