git-add--interactive works only in top level

When the working directory is not the top of the working tree, git- 
add--interactive fails silently and confusingly.  In this example,  
the working tree is rooted at ~/sandbox/foo and ~/sandbox/foo/bar/x  
is a file that's been edited.

rwbarton@homothety:~/sandbox/foo/bar$ git-add--interactive
            staged     unstaged path
   1:    unchanged        +1/-0 bar/x

*** Commands ***
   1: status       2: update       3: revert       4: add untracked
   5: patch        6: diff         7: quit         8: help
What now> 5
            staged     unstaged path
   1:    unchanged        +1/-0 bar/x
Patch update> 1

*** Commands ***
   1: status       2: update       3: revert       4: add untracked
   5: patch        6: diff         7: quit         8: help
What now>

Rather than returning to the main menu, git-add--interactive should  
have showed me a list of chunks.  It seems that the list of files is  
relative to the working tree root (which is sensible) but when git- 
add--interactive invokes git-diff-files it does not take this into  
account.  Perhaps git-diff-files should also complain when invoked as

git-diff-files -- non-existent-file-name

so it wouldn't have taken me half an hour to track this down.

Regards,
Reid Barton