update-index: use the bulk-checkin infrastructure

update-index: use the bulk-checkin infrastructure

The update-index functionality is used internally by 'git stash push' to
setup the internal stashed commit.

This change enables odb-transactions for update-index infrastructure to
speed up adding new objects to the object database by leveraging the
batch fsync functionality.

There is some risk with this change, since under batch fsync, the object
files will be in a tmp-objdir until update-index is complete, so callers
using the --stdin option will not see them until update-index is done.
This risk is mitigated by flushing the ODB transaction prior to
reporting any verbose output so that objects will be visible to callers
that are synchronizing with update-index by snooping its output.

Signed-off-by: Neeraj Singh <neerajsi@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>