Merge branch 'ps/odb-in-memory'

Add a new odb "in-memory" source that is meant to only hold tentative objects (like the virtual blob object that represents the working tree file used by "git blame"). * ps/odb-in-memory: 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