commit-graph write: more descriptive "writing out" output

commit-graph write: more descriptive "writing out" output

Make the "Writing out" part of the progress output more
descriptive. Depending on the shape of the graph we either make 3 or 4
passes over it.

Let's present this information to the user in case they're wondering
what this number, which is much larger than their number of commits,
has to do with writing out the commit graph. Now e.g. on linux.git we
emit:

    $ ~/g/git/git --exec-path=$HOME/g/git -C ~/g/linux commit-graph write
    Finding commits for commit graph: 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.

A note on i18n: Why are we using the Q_() function and passing a
number & English text for a singular which'll never be used? Because
the plural rules of translated languages may not match those of
English, and to use the plural function we need to use this format.

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