Merge branch 'ps/object-read-stream'

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`