progress: add a lock to prepare for introducing a thread

progress: move update check into helper method

progress: move current topic to member variable

progress: add try/finally to make the diffs for the next commit more readable No change in behavior.

transaction: include backup file in the "undo" transaction Once the transaction is closed, we now write transaction related data for possible future undo. For now, we only do it for full file "backup" because their were not handle at all in that case. In the future, we could move all the current logic to set undo up (that currently exists in localrepository) inside transaction itself, but it is not strictly requires to solve the current situation.

transaction: pass the name of the "undo" journal to the transaction It is time for the transaction to be responsible for setting up the undo data. It is necessary to move this logic into the transaction because many more files are handled now, and the transaction is the object tracking them all. The value can be set to None if no undo should be set.

rollback: have an empty entry for the vfsmap in rollback This empty string key is used for the store. This will be needed to properly rollback backup in a future changesets.

transaction: clarify the name of 'journal' argument for transaction The argument is a string containing the journal name (used as prefix for all other transaction file). This is not the transaction file itself. So we clarify this.