Remove the two write-only fields executable and symlink from struct
tree_entry_list. Also replace usage of the field directory with
S_ISDIR checks on the mode field, and then remove this now obsolete
field, too. Noticed by David Kastrup.
Signed-off-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx> Signed-off-by: Junio C Hamano <gitster@pobox.com>