diff: remove often unused parameters from diff_unmerge()

diff: remove often unused parameters from diff_unmerge()

e9c8409 (diff-index --cached --raw: show tree entry on the LHS for
unmerged entries., 2007-01-05) added a <mode, object name> pair as
parameters to this function, to store them in the pre-image side of an
unmerged file pair.  Now the function is fixed to return the filepair it
queued, we can make the caller on the special case codepath to do so.

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