upgrade: add '-' in optimization name The older name `redeltaall` was hard to type and read. The newer form should be more user-friendly. We keep backward compatibility with the old form (at least for a while). Having to use different form depending on the version is very impractical and error prone.

revlog: add test case for _findsnapshots Testing the method directly is more robust.

revlog: use the native implementation of issnapshot In some sparserevlog case where a lot of the history has to be searched for a snapshot, the cost of issnashot cost becomes significant. The computation done by the method is fairly low level, a native implementation provide a very significant speedup. example affected manifest write before: 0.490375s after: 0.114989s (-76%)

revlog: add a native implementation of issnapshot This will be used in the next changesets

revlog: more efficient implementation for issnapshot We avoid multiple method calls and tuple creation, this provides a significant speedup in some case: example affected manifest write before: 0.815520s after: 0.487767s (-40%)

revlog: clarify the non sparse-revlog case in `issnapshot` If we are not using sparse-revlog, there is only one type of snapshot, the full one.

exthelper: switch to using the registrar merging method

registrar: add a method to merge registrar instances This provides sanity checking beyond simply merging the underlying dictionaries.