commit-graph: fix "Writing out commit graph" progress counter

commit-graph: fix "Writing out commit graph" progress counter

76ffbca71a (commit-graph: write Bloom filters to commit graph file,
2020-04-06) added two delayed progress lines to writing the Bloom
filter index and data chunk.  This is wrong, because a single common
progress is used while writing all chunks, which is not updated while
writing these two new chunks, resulting in incomplete-looking "done"
lines:

  Expanding reachable commits in commit graph: 888679, done.
  Computing commit changed paths Bloom filters: 100% (888678/888678), done.
  Writing out commit graph in 6 passes:  66% (3554712/5332068), done.

Use the common 'struct progress' instance while writing the Bloom
filter chunks as well.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>