transaction: mark backup-related attributes private As the transaction is gaining more functions and attributes, it is important to clarify what is part of the public API.

transaction: document the contents of `tr.backupentries` Now that all items are known we can document it.

transaction: drop the third item in `tr.backupentries` This third item is always None and never used.

rebase: fix rebase with no common ancestors (issue4446) The new rebase revset didn't check for the case when there are no common ancestors. Now it does. The new behavior should be the same as the pre-3.2 behavior. Added a test.