entry: show finer-grained counter in "Filtering content" progress line

entry: show finer-grained counter in "Filtering content" progress line

The "Filtering content" progress in entry.c:finish_delayed_checkout()
is unusual because of how it calculates the progress count and because
it shows the progress of a nested loop.  It works basically like this:

  start_delayed_progress(p, nr_of_paths_to_filter)
  for_each_filter {
      display_progress(p, nr_of_paths_to_filter - nr_of_paths_still_left_to_filter)
      for_each_path_handled_by_the_current_filter {
          checkout_entry()
      }
  }
  stop_progress(p)

There are two issues with this approach:

  - The work done by the last filter (or the only filter if there is
    only one) is never counted, so if the last filter still has some
    paths to process, then the counter shown in the "done" progress
    line will not match the expected total.

    The partially-RFC series to add a GIT_TEST_CHECK_PROGRESS=1
    mode[1] helps spot this issue. Under it the 'missing file in
    delayed checkout' and 'invalid file in delayed checkout' tests in
    't0021-conversion.sh' fail, because both use only one
    filter.  (The test 'delayed checkout in process filter' uses two
    filters but the first one does all the work, so that test already
    happens to succeed even with GIT_TEST_CHECK_PROGRESS=1.)

  - The progress counter is updated only once per filter, not once per
    processed path, so if a filter has a lot of paths to process, then
    the counter might stay unchanged for a long while and then make a
    big jump (though the user still gets a sense of progress, because
    we call display_throughput() after each processed path to show the
    amount of processed data).

Move the display_progress() call to the inner loop, right next to that
checkout_entry() call that does the hard work for each path, and use a
dedicated counter variable that is incremented upon processing each
path.

After this change the 'invalid file in delayed checkout' in
't0021-conversion.sh' would succeed with the GIT_TEST_CHECK_PROGRESS=1
assertion discussed above, but the 'missing file in delayed checkout'
test would still fail.

It'll fail because its purposefully buggy filter doesn't process any
paths, so we won't execute that inner loop at all, see [2] for how to
spot that issue without GIT_TEST_CHECK_PROGRESS=1. It's not
straightforward to fix it with the current progress.c library (see [3]
for an attempt), so let's leave it for now.

Let's also initialize the *progress to "NULL" while we're at it. Since
7a132c628e5 (checkout: make delayed checkout respect --quiet and
--no-progress, 2021-08-26) we have had progress conditional on
"show_progress", usually we use the idiom of a "NULL" initialization
of the "*progress", rather than the more verbose ternary added in
7a132c628e5.

1. https://lore.kernel.org/git/20210620200303.2328957-1-szeder.dev@gmail.com/
2. http://lore.kernel.org/git/20210802214827.GE23408@szeder.dev
3. https://lore.kernel.org/git/20210620200303.2328957-7-szeder.dev@gmail.com/

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>