commit-graph: increment progress indicator

This fixes a bug that was introduced by 368d19b (commit-graph:
refactor compute_topological_levels(), 2023-03-20): Previously, the
progress indicator was updated from `i + 1` where `i` is the loop
variable of the enclosing `for` loop. After this patch, the update used
`info->progress_cnt + 1` instead, however, unlike `i`, the
`progress_cnt` attribute was not incremented. Let's increment it.

Signed-off-by: Derrick Stolee <derrickstolee@github.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
[jc: squashed in a test update from Patrick Steinhardt]
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: derrickstolee

commit-graph.c

@@ -1487,7 +1487,7 @@ static void compute_reachable_generation_numbers(
 		timestamp_t gen;
 		repo_parse_commit(info->r, c);
 		gen = info->get_generation(c, info->data);
-		display_progress(info->progress, info->progress_cnt + 1);
+		display_progress(info->progress, ++info->progress_cnt);
 
 		if (gen != GENERATION_NUMBER_ZERO && gen != GENERATION_NUMBER_INFINITY)
 			continue;

t/t6500-gc.sh

@@ -174,7 +174,7 @@ test_expect_success TTY 'with TTY: gc --no-quiet' '
 		git -c gc.writeCommitGraph=true gc --no-quiet >stdout 2>stderr &&
 	test_must_be_empty stdout &&
 	test_i18ngrep "Enumerating objects" stderr &&
-	test_i18ngrep "Computing commit graph generation numbers" stderr
+	test_i18ngrep "Computing commit graph generation numbers: 100% (4/4), done." stderr
 '
 
 test_expect_success 'gc --quiet' '