object-file: relocate ODB transaction code

The bulk-checkin subsystem provides various functions to manage ODB
transactions. Apart from {begin,end}_odb_transaction(), these functions
are only used by the object-file subsystem to manage aspects of a
transaction implementation specific to the files object source.

Relocate all the transaction code in bulk-checkin to object-file. This
simplifies the exposed transaction interface by reducing it to only
{begin,end}_odb_transaction(). Function and type names are adjusted in
the subsequent commit to better fit the new location.

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

meson.build

@@ -287,7 +287,6 @@ libgit_sources = [
   'blob.c',
   'bloom.c',
   'branch.c',
-  'bulk-checkin.c',
   'bundle-uri.c',
   'bundle.c',
   'cache-tree.c',