commit-graph verify: add progress output

commit-graph verify: add progress output

For the reasons explained in the "commit-graph write: add progress
output" commit leading up to this one, emit progress on "commit-graph
verify". Since e0fd51e1d7 ("fsck: verify commit-graph", 2018-06-27)
"git fsck" has called this command if core.commitGraph=true, but
there's been no progress output to indicate that anything was
different. Now there is (on my tiny dotfiles.git repository):

    $ git -c core.commitGraph=true -C ~/ fsck
    Checking object directories: 100% (256/256), done.
    Checking objects: 100% (2821/2821), done.
    dangling blob 5b8bbdb9b788ed90459f505b0934619c17cc605b
    Verifying commits in commit graph: 100% (867/867), done.

And on a larger repository, such as the 2015-04-03-1M-git.git test
repository:

    $ time git -c core.commitGraph=true -C ~/g/2015-04-03-1M-git/ commit-graph verify
    Verifying commits in commit graph: 100% (1000447/1000447), done.
    real    0m7.813s
    [...]

Since the "commit-graph verify" subcommand is never called from "git
gc", we don't have to worry about passing some some "report_progress"
progress variable around for this codepath.

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