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=204924Signed-off-by: NAdam Spiers <git@adamspiers.org>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>