Merge branch 'ew/prune-with-missing-objects-pack'

"git prune" may try to iterate over .git/objects/pack for trash
files to remove in it, and loudly fail when the directory is
missing, which is not necessary.  The command has been taught to
ignore such a failure.

* ew/prune-with-missing-objects-pack:
  prune: quiet ENOENT on missing directories