scmutil: add a cleanupnodes method for developers
It's now common that an old node gets replaced by zero or more new nodes,
that could happen with amend, rebase, histedit, etc. And it's a common
requirement to do bookmark movements, strip or obsolete nodes and even
moving working copy parent.
Previously, amend, rebase, history have their own logic doing the above.
This patch is an attempt to unify them and future code.
This enables new developers to be able to do "replace X with Y" thing
correctly, without any knowledge about bookmarks, strip or obsstore.
The next step will be migrating rebase to the new API, so it works inside a
transaction, and its code could be simplified.