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

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()`