Fix some bugs and make --continue handling more robust.

Fix some bugs and make --continue handling more robust.

* chdir_to_production incorrectly changed your directory
  to the *working copy* if an upgrade was in progress; this
  effectively meant that Wizard would fail to restore
  your backups in the event of an upgrade failure.  Fixing
  this also meant this function went from util to command.

* Rewrote the current directory handling for --continue, so
  that the invariant "your top-level current directory is
  the production copy" is preserved.

* Change the semantics of .wizard/pending, so that we have
  some amount of "locking" on a copy if a pending file is
  less than a day old.  Remove this file when the install
  is successful.

Signed-off-by: Edward Z. Yang <ezyang@mit.edu>