"git fsck --connectivity-only" omits computation necessary to sift
the objects that are not reachable from any of the refs into
unreachable and dangling.  This is now enabled when dangling
objects are requested (which is done by default, but can be
overridden with the "--no-dangling" option).

* jk/fsck-doc:
  fsck: always compute USED flags for unreachable objects
  doc/fsck: clarify --connectivity-only behavior