Merge branch 'tb/repack-simplify'

Simplify the logic to deal with a repack operation that ended up
creating the same packfile.

* tb/repack-simplify:
  builtin/repack.c: don't move existing packs out of the way
  builtin/repack.c: keep track of what pack-objects wrote
  repack: make "exts" array available outside cmd_repack()