bundle2: make it possible have a global transaction for the unbundling We use the `gettransaction` method approach already used for pull. We need this because we do not know beforehand if the bundle needs a transaction to be created. And (1) we do not want to create a transaction for nothing. (2) Some bundle2 bundles may be read-only and do not require any lock or transaction to be held.

tests: fix test output typo

bundle2: first crude version of bundling changeset with bundle2 The current changegroup format is put in a "changegroup" part and processed by an appropriate handlers. This is not production ready code, but let us start smoke testing.

bundle2: record processing results in the bundleoperation object Part handlers can now add records to the `bundleoperation` object. This can be used to help other parts or to let the caller of the unbundling process react to the results.