convert: display progress for filtered objects that have been delayed

convert: display progress for filtered objects that have been delayed

In 2841e8f ("convert: add "status=delayed" to filter process protocol",
2017-06-30) we taught the filter process protocol to delayed responses.
These responses are processed after the "Checking out files" phase.
If the processing takes noticeable time, then the user might think Git
is stuck.

Display the progress of the delayed responses to let the user know that
Git is still processing objects. This works very well for objects that
can be filtered quickly. If filtering of an individual object takes
noticeable time, then the user might still think that Git is stuck.
However, in that case the user would at least know what Git is doing.

It would be technical more correct to display "Checking out files whose
content filtering has been delayed". For brevity we only print
"Filtering content".

The finish_delayed_checkout() call was moved below the stop_progress()
call in unpack-trees.c to ensure that the "Checking out files" progress
is properly stopped before the "Filtering content" progress starts in
finish_delayed_checkout().

Signed-off-by: Lars Schneider <larsxschneider@gmail.com>
Suggested-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>