Merge branch 'ps/fix-geom-repack-with-alternates'

Merge branch 'ps/fix-geom-repack-with-alternates'

Geometric repacking ("git repack --geometric=<n>") in a repository
that borrows from an alternate object database had various corner
case bugs, which have been corrected.

* ps/fix-geom-repack-with-alternates:
  repack: disable writing bitmaps when doing a local repack
  repack: honor `-l` when calculating pack geometry
  t/helper: allow chmtime to print verbosely without modifying mtime
  pack-objects: extend test coverage of `--stdin-packs` with alternates
  pack-objects: fix error when same packfile is included and excluded
  pack-objects: fix error when packing same pack twice
  pack-objects: split out `--stdin-packs` tests into separate file
  repack: fix generating multi-pack-index with only non-local packs
  repack: fix trying to use preferred pack in alternates
  midx: fix segfault with no packs and invalid preferred pack