The "git_istream" abstraction has been revamped to make it easier
to interface with pluggable object database design.
* ps/object-read-stream:
streaming: drop redundant type and size pointers
streaming: move into object database subsystem
streaming: refactor interface to be object-database-centric
streaming: move logic to read packed objects streams into backend
streaming: move logic to read loose objects streams into backend
streaming: make the `odb_read_stream` definition public
streaming: get rid of `the_repository`
streaming: rely on object sources to create object stream
packfile: introduce function to read object info from a store
streaming: move zlib stream into backends
streaming: create structure for filtered object streams
streaming: create structure for packed object streams
streaming: create structure for loose object streams
streaming: create structure for in-core object streams
streaming: allocate stream inside the backend-specific logic
streaming: explicitly pass packfile info when streaming a packed object
streaming: propagate final object type via the stream
streaming: drop the `open()` callback function
streaming: rename `git_istream` into `odb_read_stream`
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