graph: fix ahead-behind logic

When we see PARENT1, it means there is a local commit and thus we are
ahead. Likewise, seeing PARENT2 means that the upstream branch has a
commit and we are one more behind.

The logic is currently reversed. Correct it.

This fixes #2501.

src/graph.c

@@ -124,9 +124,9 @@ static int ahead_behind(git_commit_list_node *one, git_commit_list_node *two,
 			(commit->flags & (PARENT1 | PARENT2)) == (PARENT1 | PARENT2))
 			continue;
 		else if (commit->flags & PARENT1)
-			(*behind)++;
-		else if (commit->flags & PARENT2)
 			(*ahead)++;
+		else if (commit->flags & PARENT2)
+			(*behind)++;
 
 		for (i = 0; i < commit->out_degree; i++) {
 			git_commit_list_node *p = commit->parents[i];