make gotadmin cleanup pack the repository before cleaning
Our cleanup implementation is only safe to use after everything
referenced has been packed into a single pack file. Otherwise, the
algorithm we use for checking pack redundancy might remove small
packs which contain objects that other objects depend on.
The easy fix for this issue is to have 'gotadmin cleanup' create the
required pack file before cleaning up, making cleanup safe by default.
This happens to be what 'git gc' does as well.
ok jamsek