Start adopting clearer names for exclude functions. This 'is_*'
naming pattern for functions returning booleans was agreed here:
http://thread.gmane.org/gmane.comp.version-control.git/204661/focus=204924
Signed-off-by: Adam Spiers <git@adamspiers.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>