bulk-checkin: rename 'state' variable and separate 'plugged' boolean

bulk-checkin: rename 'state' variable and separate 'plugged' boolean

This commit prepares for adding batch-fsync to the bulk-checkin
infrastructure.

The bulk-checkin infrastructure is currently used to batch up addition
of large blobs to a packfile. When a blob is larger than
big_file_threshold, we unconditionally add it to a pack. If bulk
checkins are 'plugged', we allow multiple large blobs to be added to a
single pack until we reach the packfile size limit; otherwise, we simply
make a new packfile for each large blob. The 'unplug' call tells us when
the series of blob additions is done so that we can finish the packfiles
and make their objects available to subsequent operations.

Stated another way, bulk-checkin allows callers to define a transaction
that adds multiple objects to the object database, where the object
database can optimize its internal operations within the transaction
boundary.

Batched fsync will fit into bulk-checkin by taking advantage of the
plug/unplug functionality to determine the appropriate time to fsync
and make newly-added objects available in the primary object database.

* Rename 'state' variable to 'bulk_checkin_packfile', since we will
  later be adding 'bulk_fsync_objdir'. This also makes the variable
  easier to find in the debugger, since the name is more unique.

* Rename finish_bulk_checkin to flush_bulk_checkin_packfile and call it
  unconditionally from unplug_bulk_checkin. Internally it will
  conditionally do a flush if there's any work to do.

* Move the 'plugged' data member of 'bulk_checkin_state' into a separate
  static variable. Doing this avoids resetting the variable in
  finish_bulk_checkin when zeroing the 'bulk_checkin_state'. As-is, we
  seem to unintentionally disable the plugging functionality the first
  time a new packfile must be created due to packfile size limits. While
  disabling the plugging state only results in suboptimal behavior for
  the current code, it would be fatal for the bulk-fsync functionality
  later in this patch series.

The net effect of these changes is to make a clear separation between
the portion of the bulk-checkin infrastructure that is related to the
packfile (nearly all of it at present) and the part that is related to
other future optimizations of the ODB.

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