git-{diff,ls}-files from a subdirectory fails ...

... when GIT_DIR is specified.

I have a repository at ~/stgit-lib. With cwd at the top of the tree, things work as expected when I set all those fancy envoronment variables:

  kha@yoghurt:~/stgit-lib> git-diff-files --name-only
  stgit/utillib.py

  kha@yoghurt:~/stgit-lib> GIT_DIR=/home/kha/stgit-lib/.git GIT_INDEX_FILE=/home/kha/stgit-lib/.git/index GIT_WORK_TREE=/home/kha/stgit-lib git-diff-files --name-only
  stgit/utillib.py

However, it doen't seem to work from a subdirectory:

  kha@yoghurt:~/stgit-lib/stgit> git-diff-files --name-only
  stgit/utillib.py

  kha@yoghurt:~/stgit-lib/stgit> GIT_DIR=/home/kha/stgit-lib/.git GIT_INDEX_FILE=/home/kha/stgit-lib/.git/index GIT_WORK_TREE=/home/kha/stgit-lib git-diff-files --name-only | wc
      170     170    3560

Instead of just the one changed file, I get a list of what looks like all the files in the project, suggesting that maybe git assumes I'm at the root of the worktree when I'm not.

git-ls-files seems to behave the same way. Bug or user error?