revlog: do not search for delta for empty content We won't find any useful base to store an empty text. So we should not even try to.

revlog: split and document good delta conditional The logic is still identical, but having each conditional on its own helps to document them and will help to edit them in the future.

revlog: skip over empty revision when looking for delta base There is no point in keeping an empty revision in the middle of a chain since they won't contribute any data. So we filter them out when looking for a valid base delta.

debugrevlog: display details about empty revision We now display details about the course of emptiness (empty text or empty delta).

debugrevlog: track empty revlog entries A revlog entry can be "empty" for two reasons: * the stored text is empty * we found another entry with the exact same content Tracking them separately make revlog content clearer.

debugrevlog: document some of the variable used This help to understand the code.

revlog: drop compatibility for `revlog.descendant` (API) As advertised, the method is dropped in 4.8.

context: drop compatibility for `context.descendant` (API) As advertised, the method is dropped in 4.8.

cmdutil: remove _updatecleanmsg() which is no longer used anywhere

cmdutil: fix weird indent in _mergemsg()