dir.c: rename excluded() to is_excluded()

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

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

attr.c

@@ -284,7 +284,7 @@ static struct match_attr *parse_attr_line(const char *line, const char *src,
  * (reading the file from top to bottom), .gitattribute of the root
  * directory (again, reading the file from top to bottom) down to the
  * current directory, and then scan the list backwards to find the first match.
- * This is exactly the same as what excluded() does in dir.c to deal with
+ * This is exactly the same as what is_excluded() does in dir.c to deal with
  * .gitignore
  */