Merge branch 'dk/gc-idx-wo-pack' into maint

Merge branch 'dk/gc-idx-wo-pack' into maint

Having a leftover .idx file without corresponding .pack file in
the repository hurts performance; "git gc" learned to prune them.

We may want to do the same for .bitmap (and notice but not prune
.keep) without corresponding .pack, but that can be a separate
topic.

* dk/gc-idx-wo-pack:
  gc: remove garbage .idx files from pack dir
  t5304: test cleaning pack garbage
  prepare_packed_git(): refactor garbage reporting in pack directory