The new option allows prune to operate with nearly no scratch space by only removing
no longer necessary pack files and first deleting the index before
rebuilding it. By first deleting the index it becomes safe to just
delete no longer necessary pack files. However, as a downside there's
now the risk that the repository becomes inaccessible if prune fails.
To recover from that problem a user might have to manually delete the
repository index and then run (a full) `rebuild-index` again.
Michael Eischer