Gitk "External diff" broken when using worktree

Hi,

I'm using git 2.7.3 on cygwin, and have been taking advantage of the new "git worktree" feature. I noticed that when I launch gitk from one of the attached working directories, its "external diff" feature doesn't seem to work. Nothing shows up in the diff tool at all. However, it works if you launch gitk from the main repository.

To reproduce:
1. Create a new working tree with "git worktree add"
2. From that new worktree, launch gitk.
3. Right-click in a file in the bottom right pane and click "External diff"
4. No useful diff appears.