debugrevlog: fix computing chain length in debugrevlog -d The chain length was computed correctly only when generaldelta feature was enabled. Now it's fixed. When generaldelta is disabled the base revision in revlog index is not the revision we have delta against - it's always previous revision. Instead of incorrect chainbaseandlen in command.py we are now using two single-responsibility functions in revlog.py: - chainbase(rev) - chainlen(rev) Only chainlen(rev) was missing so it was written to mimic the way the chain of deltas is actually found during file reconstruction.

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".

transaction: document startgroup and endgroup These enigmatic methods are only used in repair. We document them to clarify there purpose and user.

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.