When a merge left unmerged entries, git add failed to pick up the
file mode from the index, when core.filemode == 0. If more than one
unmerged entry is there, the order of stage preference is 2, 1, 3.

Noticed by Johannes Sixt.
Signed-off-by: NJohannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>