rebase: small refactoring to allow better extensibility from extensions
Inhibit, one of evolve's extension, would like to change the way rebase works
with obsolete changesets. During a rebase with inhibit, the inhibition of the
obsolescence markers should be lifted for the rebase.
With this small refactoring, inhibit and can wrap the _filterobsoleterevs
function to lift inhibition cleanly and at the same time this change makes
rebases' code more legible.