Merge branch 'ps/object-store-midx-dedup-info' into ps/packfile-store

Merge branch 'ps/object-store-midx-dedup-info' into ps/packfile-store

* ps/object-store-midx-dedup-info:
  midx: compute paths via their source
  midx: stop duplicating info redundant with its owning source
  midx: write multi-pack indices via their source
  midx: load multi-pack indices via their source
  midx: drop redundant `struct repository` parameter
  odb: simplify calling `link_alt_odb_entry()`
  odb: return newly created in-memory sources
  odb: consistently use "dir" to refer to alternate's directory
  odb: allow `odb_find_source()` to fail
  odb: store locality in object database sources