progress: add sparse mode to force 100% complete message

progress: add sparse mode to force 100% complete message

Add new start_sparse_progress() and start_delayed_sparse_progress()
constructors and "sparse" flag to struct progress.

Teach stop_progress() to force a 100% complete progress message before
printing the final "done" message when "sparse" is set.

Calling display_progress() for every item in a large set can
be expensive.  If callers try to filter this for performance
reasons, such as emitting every k-th item, progress would
not reach 100% unless they made a final call to display_progress()
with the item count before calling stop_progress().

Now this is automatic when "sparse" is set.

Signed-off-by: Jeff Hostetler <jeffhost@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>