Merge branch 'ps/odb-in-memory' into ps/odb-source-loose

* ps/odb-in-memory: (24 commits) t/unit-tests: add tests for the in-memory object source odb: generic in-memory source odb/source-inmemory: stub out remaining functions odb/source-inmemory: implement `freshen_object()` callback odb/source-inmemory: implement `count_objects()` callback odb/source-inmemory: implement `find_abbrev_len()` callback odb/source-inmemory: implement `for_each_object()` callback odb/source-inmemory: convert to use oidtree oidtree: add ability to store data cbtree: allow using arbitrary wrapper structures for nodes odb/source-inmemory: implement `write_object_stream()` callback odb/source-inmemory: implement `write_object()` callback odb/source-inmemory: implement `read_object_stream()` callback odb/source-inmemory: implement `read_object_info()` callback odb: fix unnecessary call to `find_cached_object()` odb/source-inmemory: implement `free()` callback odb: introduce "in-memory" source odb/transaction: make `write_object_stream()` pluggable object-file: generalize packfile writes to use odb_write_stream object-file: avoid fd seekback by checking object size upfront ...