Merge branch 'ps/odb-alternates-object-sources' into next

Code refactoring around alternate object store.

* ps/odb-alternates-object-sources:
  odb: write alternates via sources
  odb: read alternates via sources
  odb: drop forward declaration of `read_info_alternates()`
  odb: remove mutual recursion when parsing alternates
  odb: stop splitting alternate in `odb_add_to_alternates_file()`
  odb: move computation of normalized objdir into `alt_odb_usable()`
  odb: resolve relative alternative paths when parsing
  odb: refactor parsing of alternates to be self-contained

Author: gitster