commit-graph: split up close_reachable() progress output

commit-graph: split up close_reachable() progress output

Amend the progress output added in 7b0f229222 ("commit-graph write:
add progress output", 2018-09-17) so that the total numbers it reports
aren't higher than the total number of commits anymore. See [1] for a
bug report pointing that out.

When I added this I wasn't intending to provide an accurate count, but
just have some progress output to show the user the command wasn't
hanging[2]. But since we are showing numbers, let's make them
accurate. The progress descriptions were suggested by Derrick Stolee
in [3].

As noted in [2] we are unlikely to show anything except the "Expanding
reachable..." message even on fairly large repositories such as
linux.git. On a test repository I have with north of 7 million commits
all of these are displayed. Two of them don't show up for long, but as
noted in [5] future-proofing this for if the loops become more
expensive in the future makes sense.

1. https://public-inbox.org/git/20181010203738.GE23446@szeder.dev/
2. https://public-inbox.org/git/87pnwhea8y.fsf@evledraar.gmail.com/
3. https://public-inbox.org/git/f7a0cbee-863c-61d3-4959-5cec8b43c705@gmail.com/
4. https://public-inbox.org/git/20181015160545.GG19800@szeder.dev/
5. https://public-inbox.org/git/87murle8da.fsf@evledraar.gmail.com/

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