using rev-list to tell if a branch is behind or ahead

I'm implementing a ruby interface to git and am wanting to be able to ask if a branch is ahead or behind.

I looked in the builtin-checkout.c file and see this code:

	/* Run "rev-list --left-right ours...theirs" internally... */
	rev_argc = 0;
	rev_argv[rev_argc++] = NULL;
	rev_argv[rev_argc++] = "--left-right";
	rev_argv[rev_argc++] = symmetric;
	rev_argv[rev_argc++] = "--";
	rev_argv[rev_argc] = NULL;

	strcpy(symmetric, sha1_to_hex(ours->object.sha1));
	strcpy(symmetric + 40, "...");
	strcpy(symmetric + 43, sha1_to_hex(theirs->object.sha1));

	init_revisions(&revs, NULL);
	setup_revisions(rev_argc, rev_argv, &revs, NULL);
	prepare_revision_walk(&revs);

	/* ... and count the commits on each side. */
	num_ours = 0;
	num_theirs = 0;
	while (1) {
		struct commit *c = get_revision(&revs);
		if (!c)
			break;
		if (c->object.flags & SYMMETRIC_LEFT)
			num_ours++;
		else
			num_theirs++;
	}

It looks like it's calling rev-parse. But, when I call it with the same arguments (using branches or commit sha1's), it only will list commits that are in right and not in left. I need it to show both ways: commits that are in the right and not in left, and commits that are in the left but not in right.

Do I need to call rev-parse twice to achieve this?

Here's a sample of what I'm trying currently:
~ $ mkdir test
~ $ cd test/
~/test $ git init
Initialized empty Git repository in .git/
~/test $ git
~/test $ echo content > file.txt
~/test $ git add file.txt && git commit -m "Initial commit"
Created initial commit f5e4160: Initial commit
  1 files changed, 1 insertions(+), 0 deletions(-)
  create mode 100644 file.txt
~/test master$ git co -b task
Switched to a new branch "task"
~/test task$ echo changes >> file.txt
~/test task$ git add file.txt && git commit -m "Some changes"
Created commit 96492ee: Some changes
  1 files changed, 1 insertions(+), 0 deletions(-)
~/test task$ git rev-list --left-right task..master --
~/test task$ git rev-list --left-right master..task --
 >96492ee80143f43417b00699ff29330d0027df7f

Thanks,

Tim