gc: merge prune and gc into single step

pass --prune=all to gc to ensure that we properly cleanup unpacked and
packed objects and don't have the weird bloating effect after a first
pass, e.g.:

  $ asp disk-usage
  ==> Using 640K on disk.

  $ asp gc
  Nothing new to pack.

  $ asp disk-usage
  ==> Using 8.4M on disk.

  $ asp gc
  Nothing new to pack.

  $ asp disk-usage
  ==> Using 328K on disk.

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