commit-graph.c: display correct number of chunks when writing

commit-graph.c: display correct number of chunks when writing

When writing a commit-graph, a progress meter is shown which indicates
the number of pieces of data to write (one per commit in each chunk).

In 47410aa837 (commit-graph: use chunk-format write API, 2021-02-18),
the number of chunks became tracked by the new chunk-format API. But a
stray local variable was left behind from when write_commit_graph_file()
used to keep track of the same.

Since this was no longer updated after 47410aa837, the progress meter
appeared broken:

    $ git commit-graph write --reachable
    Expanding reachable commits in commit graph: 837569, done.
    Writing out commit graph in 3 passes: 166% (4187845/2512707), done.

Drop the local variable and rely instead on the chunk-format API to tell
us the correct number of chunks.

Reported-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Acked-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>