object-file: generalize packfile writes to use odb_write_stream

object-file: generalize packfile writes to use odb_write_stream

The `index_blob_packfile_transaction()` function streams blob data
directly from an fd. This makes it difficult to reuse as part of a
generic transactional object writing interface.

Refactor the packfile write path to operate on a `struct
odb_write_stream`, allowing callers to supply data from arbitrary
sources.

Signed-off-by: Justin Tobler <jltobler@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>