commit-graph write: add itermediate progress

commit-graph write: add itermediate progress

Add progress output to sections of code between "Annotating[...]" and
"Computing[...]generation numbers". This can collectively take 5-10
seconds on a large enough repository.

On a test repository with I have with ~7 million commits and ~50
million objects we'll now emit:

    $ ~/g/git/git --exec-path=$HOME/g/git commit-graph write
    Finding commits for commit graph among packed objects: 100% (124763727/124763727), done.
    Loading known commits in commit graph: 100% (18989461/18989461), done.
    Expanding reachable commits in commit graph: 100% (18989507/18989461), done.
    Clearing commit marks in commit graph: 100% (18989507/18989507), done.
    Counting distinct commits in commit graph: 100% (18989507/18989507), done.
    Finding extra edges in commit graph: 100% (18989507/18989507), done.
    Computing commit graph generation numbers: 100% (7250302/7250302), done.
    Writing out commit graph in 4 passes: 100% (29001208/29001208), done.

Whereas on a medium-sized repository such as linux.git these new
progress bars won't have time to kick in and as before and we'll still
emit output like:

    $ ~/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: 815990, done.
    Computing commit graph generation numbers: 100% (815983/815983), done.
    Writing out commit graph in 4 passes: 100% (3263932/3263932), done.

The "Counting distinct commits in commit graph" phase will spend most
of its time paused at "0/*" as we QSORT(...) the list. That's not
optimal, but at least we don't seem to be stalling anymore most of the
time.

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