revlog: add config variable for limiting delta-chain length The current heuristic for deciding between storing delta and full texts is based on ratio of (sizeofdeltas)/(sizeoffulltext). In some cases (for example a manifest for ahuge repo) this approach can result in extremely long delta chains (~30,000) which are very slow to read. (In the case of a manifest ~500ms are added to every hg command because of that). This commit introduces "revlog.maxchainlength" configuration variable that will limit delta chain length.

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`