dir.c: rename excluded_from_list() to is_excluded_from_list()

Continue adopting clearer names for exclude functions.  This 'is_*'
naming pattern for functions returning booleans was discussed here:

http://thread.gmane.org/gmane.comp.version-control.git/204661/focus=204924

Also adjust their callers as necessary.

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