progress.c: refactor stop_progress{,_msg}() to use helpers

progress.c: refactor stop_progress{,_msg}() to use helpers

Create two new static helpers for the stop_progress() and
stop_progress_msg() functions.

As we'll see in the subsequent commit having those two split up
doesn't make much sense, and results in a bug in how we log to
trace2. This narrow preparatory change makes the diff for that
subsequent change smaller.

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