commit-graph: fix bogus counter in "Scanning merged commits" progress line

The final value of the counter of the "Scanning merged commits"
progress line is always one less than its expected total, e.g.:

  Scanning merged commits:  83% (5/6), done.

This happens because while iterating over an array the loop variable
is passed to display_progress() as-is, but while C arrays (and thus
the loop variable) start at 0 and end at N-1, the progress counter
must end at N. Fix this by passing 'i + 1' to display_progress(), like
most other callsites do.

There's an RFC series to add a GIT_TEST_CHECK_PROGRESS=1 mode[1] which
catches this issue in the 'fetch.writeCommitGraph' and
'fetch.writeCommitGraph with submodules' tests in
't5510-fetch.sh'. The GIT_TEST_CHECK_PROGRESS=1 mode is not part of
this series, but future changes to progress.c may add it or similar
assertions to catch this and similar bugs elsewhere.

1. https://lore.kernel.org/git/20210620200303.2328957-1-szeder.dev@gmail.com/

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

commit-graph.c

@@ -2096,7 +2096,7 @@ static void sort_and_scan_merged_commits(struct write_commit_graph_context *ctx)
 
 	ctx->num_extra_edges = 0;
 	for (i = 0; i < ctx->commits.nr; i++) {
-		display_progress(ctx->progress, i);
+		display_progress(ctx->progress, i + 1);
 
 		if (i && oideq(&ctx->commits.list[i - 1]->object.oid,
 			  &ctx->commits.list[i]->object.oid)) {