borg2: check [--repair] improvements

Currently (beta12), borg2 check just deletes the chunks index cached in the repo. This is because borg check --repair might have deleted some invalid repo objects, thus the cached chunks index is not uptodate anymore. After that, first access to the repo will rebuild the chunks index cache (slow).

Guess we could just build and cache a new chunks index within borg check --repair and maybe also in borg check without --repair.