[BUG] Spurious leading '/' in filename in "git diff --raw --relative=<subdirectory>"

I wanted to get non-recursive raw diff (difftree), but for a given subdirectory and not starting from root.

I have found '--relative[=<path>]', introduced in 

  c0cb4a0 (diff --relative: help working in a bare repository, 2008-02-13)
  cd676a5 (diff --relative: output paths as relative to the current subdirectory, 2008-02-12)

But when examining it's output in 'raw' mode, I have notice spurious leading '/' in filename:

  $ git diff-tree --abbrev -r --raw HEAD --relative=sub
  a3a8425fe5496c61921010cb1e7b455a1f52bb86
  :100644 100644 d90bda0... cefcae0... M	/quux

The output without '-r' (recurse into sub-trees) is even more strange

  $ git diff-tree --abbrev --raw HEAD --relative=sub
  a3a8425fe5496c61921010cb1e7b455a1f52bb86
  :040000 040000 e62aa6e... b5d4a43... M	

(that's a trailing TAB, and no filename).

What I expected was the following output:

  $ git diff-tree --abbrev --raw HEAD --relative=sub
  a3a8425fe5496c61921010cb1e7b455a1f52bb86
  :040000 040000 e62aa6e... b5d4a43... M	quux

I see that the '--relative' and '--relative=<path>' options were introduced for patch ('-p') format, and not for difftree / raw format, but I think they should work for it, too.

P.S. I have noticed this bug when working on proof-of-concept tree-blame (i.e. when given file was modified) in Perl.