amend: allow amend of non-head when obsolete is enabled
Obsolescence marker can represent this situation just fine. The old
version is marked as precursor of the new changeset. All its
descendants become "unstable".
If obsolescence is not enabled we keep the current behavior of
aborting. This new behavior only applies when obsolete is
enabled and is subject to future discussion and changes.
basic:
creating
creating
creating
creating
fakeuncacheable:
creating
creating
creating
creating
creating
creating