debugupgraderepo: add a --no-backup mode The process has been around for a while and is pretty safe now. Having an automated way to clean up the old data is useful when running many different conversion to compare delta algorithm.

upgrade: add '-' in optimization name The older name `redeltaall` was hard to type and read. The newer form should be more user-friendly. We keep backward compatibility with the old form (at least for a while). Having to use different form depending on the version is very impractical and error prone.

revlog: add test case for _findsnapshots Testing the method directly is more robust.