obsolete: compute extinct changesets...
obsolete: compute extinct changesets
`extinct` changesets are obsolete changesets with obsolete descendants only. They
are of no interest anymore and can be:
- exclude from exchange
- hidden to the user in most situation
- safely garbage collected
This changeset just allows mercurial to detect them.
The implementation is a bit naive, as for unstable changesets. We better use a
simple revset query and a cache, but simple version comes first.