"git diff-index" broken in 'next'

When you are in the middle of conflicted merge, it is useful to view "git diff" that compares the stage #2 and your work tree (which has conflict markers and both sides of the changes). This still works.

However, with the unpack_trees() update, another form of useful sanity check seems to be totally broken:

      $ git diff HEAD -- $conflicted_file

This used to show the diff between the HEAD and the work tree, but not anymore. It shows a diff as if the whole file is gone, and that is because diff-lib.c::oneway_diff() gets idx==NULL from the caller for unmerged entries. It never reaches show_modified() codepath.

I think it is somewhere around ll. 280 in unpack_callback() that causes this, but I don't have time to dig this further for now.