check-ignore: move setup into cmd_check_ignore()

check-ignore: move setup into cmd_check_ignore()

Initialisation of the dir_struct and path_exclude_check structs was
previously done within check_ignore().  This was acceptable since
check_ignore() was only called once per check-ignore invocation;
however the next commit will convert it into an inner loop which is
called once per line of STDIN when --stdin is given.  Therefore moving
the initialisation code out into cmd_check_ignore() ensures that
initialisation is still only performed once per check-ignore
invocation, and consequently that the output is identical whether
pathspecs are provided as CLI arguments or via STDIN.

Signed-off-by: Adam Spiers <git@adamspiers.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>