diff-files: show unmerged entries correctly

diff-files: show unmerged entries correctly

Earlier, e9c8409 (diff-index --cached --raw: show tree entry on the LHS
for unmerged entries., 2007-01-05) taught the command to show the object
name and the mode from the entry coming from the tree side when comparing
a tree with an unmerged index.

This is a belated companion patch that teaches diff-files to show the mode
from the entry coming from the working tree side, when comparing an
unmerged index and the working tree.

Signed-off-by: Junio C Hamano <gitster@pobox.com>