transaction: factorise append-only file registration The addition is done in two different places but differs slightly. We factorise this addition to ensure it is consistent in all places.

transaction: document `tr.add`

transaction: drop backupentries logic from startgroup and endgroup The `startgroup` and `endgroup` methods are used in a very specific context to wrap a very specific operation (revlog truncation). It does not make sense to perform any other operations during such a "group" (eg:file backup). There is currently no user of backupfile during a "group" so we drop the group-specific code and restrict authorized operations during "group".