patchcommit-graph: fix progress of reachable commits

> To display a progress line while iterating over all refs,
> d335ce8f24 (commit-graph.c: show progress of finding reachable
> commits, 2020-05-13) should have added a pair of
> start_delayed_progress() and stop_progress() calls around a
> for_each_ref() invocation.  Alas, the stop_progress() call ended up at
> the wrong place, after write_commit_graph(), which does all the
> commit-graph computation and writing, and has several progress lines
> of its own.  Consequently, that new
>
>   Collecting referenced commits: 123
>
> progress line is overwritten by the first progress line shown by
> write_commit_graph(), and its final "done" line is shown last, after
> everything is finished:
>
>   Expanding reachable commits in commit graph: 344786, done.
>   Computing commit changed paths Bloom filters: 100% (344786/344786), done.
>   Collecting referenced commits: 154, done.
>
> Move that stop_progress() call to the right place.
>
> While at it, drop the unnecessary 'if (data.progress)' condition
> protecting the stop_progress() call, because that function is prepared
> to handle a NULL progress struct.

>
> Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
> ---
>  commit-graph.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
>
> diff --git a/commit-graph.c b/commit-graph.c
> index 5df3e08718..aaf3327ede 100644
> --- a/commit-graph.c
> +++ b/commit-graph.c
> @@ -1354,12 +1354,13 @@ int write_commit_graph_reachable(struct object_directory *odb,
>  			_("Collecting referenced commits"), 0);
>  
>  	for_each_ref(add_ref_to_set, &data);
> +
> +	stop_progress(&data.progress);
> +
>  	result = write_commit_graph(odb, NULL, &commits,
>  				    flags, split_opts);
>  
>  	oidset_clear(&commits);
> -	if (data.progress)
> -		stop_progress(&data.progress);
>  	return result;
>  }