revision: keep topo-walk free of unintersting commits

When updating the topo-order walk in b454241 (revision.c: generation-based
topo-order algorithm, 2018-11-01), the logic was a huge rewrite of the
walk logic. In that massive change, we accidentally included the
UNINTERESTING commits in expand_topo_walk(). This means that a simple
query like

    git rev-list --topo-order HEAD~1..HEAD

will expand the topo walk for all commits reachable from HEAD, and not
just one commit.

This change should speed up these cases, but there is still a need
for corrected commit-date for some A..B queries.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

revision.c

@@ -3265,6 +3265,9 @@ static void expand_topo_walk(struct rev_info *revs, struct commit *commit)
 		struct commit *parent = p->item;
 		int *pi;
 
+		if (parent->object.flags & UNINTERESTING)
+			continue;
+
 		if (parse_commit_gently(parent, 1) < 0)
 			continue;