revwalk: clear the flags on reset

These store merge-base information which is only valid for a single run.

src/revwalk.c

@@ -540,6 +540,7 @@ void git_revwalk_reset(git_revwalk *walk)
 		commit->in_degree = 0;
 		commit->topo_delay = 0;
 		commit->uninteresting = 0;
+		commit->flags = 0;
 		});
 
 	git_pqueue_clear(&walk->iterator_time);