commit-graph: use start_delayed_progress()

commit-graph: use start_delayed_progress()

When writing a commit-graph, we show progress along several commit
walks. When we use start_delayed_progress(), the progress line will
only appear if that step takes a decent amount of time.

However, one place was missed: computing generation numbers. This is
normally a very fast operation as all commits have been parsed in a
previous step. But, this is showing up for all users no matter how few
commits are being added.

The tests that check for the progress output have already been updated
to use GIT_PROGRESS_DELAY=0 to force the expected output.

Helped-by: Jeff King <peff@peff.net>
Reported-by: ryenus <ryenus@gmail.com>
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>