odb/transaction: add transaction env interface

The ODB transaction backend is responsible for creating/managing its own
staging area for writing objects. Other child processes spawned by Git
may need access to uncommitted objects or write new objects in the
staging area though.

Introduce `odb_transaction_env()` which is expected to provide the set
of environment variables needed by a child process to access the
transaction's staging area.

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

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