diff-cache: handle modified new files correctly
diff-cache: handle modified new files correctly

Junio pointed out that diff-cache didn't handle the case of a new file
that was different from its index entry correctly. It needs to check
the working copy the same way the modified file case did.

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git