There was no good way to ask "I have a random string that came from
outside world. I want to turn it into a 40-hex object name while
making sure such an object exists".  A new peeling suffix ^{object}
can be used for that purpose, together with "rev-parse --verify".

* jc/sha1-name-object-peeler:
  peel_onion(): teach $foo^{object} peeler
  peel_onion: disambiguate to favor tree-ish when we know we want a tree-ish