Recent "git prune" traverses young unreachable objects to safekeep
old objects in the reachability chain from them, which sometimes
caused error messages that are unnecessarily alarming.

* jk/squelch-missing-link-warning-for-unreachable:
  suppress errors on missing UNINTERESTING links
  silence broken link warnings with revs->ignore_missing_links
  add quieter versions of parse_{tree,commit}