commit-graph write: emit a percentage for all progress

commit-graph write: emit a percentage for all progress

Follow-up 01ca387774 ("commit-graph: split up close_reachable()
progress output", 2018-11-19) by making the progress bars in
close_reachable() report a completion percentage. This fixes the last
occurrence where in the commit graph writing where we didn't report
that.

The change in 01ca387774 split up the 1x progress bar in
close_reachable() into 3x, but left them as dumb counters without a
percentage completion. Fixing that is easy, and the only reason it
wasn't done already is because that commit was rushed in during the
v2.20.0 RC period to fix the unrelated issue of over-reporting commit
numbers. See [1] and follow-ups for ML activity at the time and [2]
for an alternative approach where the progress bars weren't split up.

Now for e.g. linux.git we'll emit:

    $ ~/g/git/git --exec-path=$HOME/g/git commit-graph write
    Finding commits for commit graph among packed objects: 100% (6529159/6529159), done.
    Expanding reachable commits in commit graph: 100% (815990/815980), done.
    Computing commit graph generation numbers: 100% (815983/815983), done.
    Writing out commit graph in 4 passes: 100% (3263932/3263932), done.

1. https://public-inbox.org/git/20181119202300.18670-1-avarab@gmail.com/
2. https://public-inbox.org/git/20181122153922.16912-11-avarab@gmail.com/

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