git ls-tree -d doesn't work if the specified path is the repository root?

Hi,

When I run `git ls-tree -d HEAD -- subdir` from the root of my repository, where `subdir` is a subdirectory in that root, I get the treehash of that subdirectory. This is what I expect.

However, if I merely replace `subdir` with `.` (the root of the repository), (i.e., `git ls-tree -d HEAD -- .`) git ls-tree returns the treehashes of the /children/ of the root, instead of the root itself, contrary to the documented behavior of -d.

Is there some reason for this? This behavior seems like a bug to me: it means that prior to calling ls-tree I need to check if the referenced path happens to be the root, and if so, find some other means (rev-parse?) of converting it to a treehash.

Thanks, —Roy