bulk-checkin: introduce object database transaction structure

bulk-checkin: introduce object database transaction structure

Object database transaction state is stored across several global
variables in the bulk-checkin subsystem. Consolidate this state into a
single `struct odb_transaction` global. In a subsequent commit, the
transactional interfaces will be updated to wire this structure instead
of relying on a global variable.

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