Like before, absorb continues to not touch the working directory, so any
working directory changes are preserved, even if the working directory parent
changes underneath (for instance, because the current revision was replaced
when changes were absorbed into it).
This can also be combined with --interactive to absorb just part of a
commit into its parents.
This initial implementation has the shortcoming that it does not do anything
with the absorbed commit:
- With obsmarkers, this means the user still needs to evolve, rebase or prune manually
- Without obsmarkers, the old commits would not be stripped at all because their child was not replaced