Merge branch 'ps/odb-move-loose-object-writing'

Merge branch 'ps/odb-move-loose-object-writing'

The logic to write loose objects has been refactored and moved from
'object-file.c' to the loose backend source file 'odb/source-loose.c',
making the loose backend more self-contained.  This is achieved by
first refactoring force_object_loose() to use generic ODB write
interfaces instead of loose-backend internals.

* ps/odb-move-loose-object-writing:
  object-file: move logic to write loose objects
  object-file: move `force_object_loose()`
  object-file: force objects loose via generic interface
  object-file: fix memory leak in `force_object_loose()`
  odb: support setting mtime when writing objects
  odb: lift object existence check out of the "loose" backend
  odb: compute object hash in `odb_write_object_ext()`
  t/u-odb-inmemory: implement wrapper for writing objects
  odb: compute compat object ID in `odb_write_object_ext()`