progress API: unify stop_progress{,_msg}(), fix trace2 bug

progress API: unify stop_progress{,_msg}(), fix trace2 bug

Fix a bug that's been with us ever since 98a13647408 (trace2: log
progress time and throughput, 2020-05-12), when the
stop_progress_msg() API was used we didn't log a "region_leave" for
the "region_enter" we start in "start_progress_delay()".

The only user of the "stop_progress_msg()" function is
"index-pack". Let's add a previously failing test to check that we
have the same number of "region_enter" and "region_leave" events, with
"-v" we'll log progress even in the test environment.

In addition to that we've had a submarine bug here introduced with
9d81ecb52b5 (progress: add sparse mode to force 100% complete message,
2019-03-21). The "start_sparse_progress()" API would only do the right
thing if the progress was ended with "stop_progress()", not
"stop_progress_msg()".

The only user of that API uses "stop_progress()", but let's still fix
that along with the trace2 issue by making "stop_progress()" a trivial
wrapper for "stop_progress_msg()".

We can also drop the "if (progress)" test from
"finish_if_sparse()". It's now a helper for the small
"stop_progress_msg()" function. We'll already have returned from it if
"progress" is "NULL".

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