This wraps the cleaning up of loose objects and redundant pack files under a new functions, making the _prepare() and _complete() functions unnecessary. It walks the reachable commits unconditionally since that information is always needed and adapt how we purge loose objects after this change. The progress function is changed too and we log `ncommits' first, followed by the number of loose objects, followed by the rest. Pack files are now considered redundant when all their objects are provided by a bigger pack or are unreachable. ok stsp

This adds the functions got_repo_cleanup_prepare() and _complete() to lock/unlock the repository to prevent multiple `gotadmin cleanup' or `git gc' operations to run in parallel. improvements and ok stsp@

improvements and ok stsp@

Intended mostly for the regress suite, so we'll be able to test also the ref-delta code paths. ok stsp@

ok op@

ok millert