This option allow a strict set of revision to be specified instead of using -s
or -b. Rebase will refuse start if striping rebased changeset will strip non
rebased changeset. Rebase will refuse to work on set with multiple root.
The buildstate function now take a set of revs. Logic related to --source and
--base option have been moved in the main rebase function.
In the process this fixes a bug where the wrong source changeset might be pick.
This explain the changes in hgext/rebase.py
We only deny rebasing onto direct parent. Thanks to the ancestor argument of
merge. the "implementation" of this feature only consist in loosing the check
and imply detach when rebasing on ancestor.
