Code review comment for lp:~abentley/bzr/is_executable

I proposed for 2.2 because I want it for Launchpad, and because it seemed like a small change. I didn't imagine that clients would actually care about the executability of symlinks or directories, but I can think of at least one client (difftacular, used by Launchpad) that cares that the output of RevisionTree.iter_changes(PreviewTree) is sane.

So on balance, it still seems like a reasonable change to make in a stable series. That said, I can propose a more conservative patch for 2.2 that will merely correct the output of RevisionTree.iter_changes(PreviewTree). Or I guess I could fix in in difftactular, though I'd rather not.