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()`
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