ls-files --others --directory: fix a bug with index entry ordering

ls-files --others --directory: fix a bug with index entry ordering

When both howto-index.sh and howto/make-dist.txt exist under
Documentation/ directory, dir_exists() mistakenly checked it
without the trailing slash to see if there was something under
Documentation/howto directory, and did not realize there was,
because '-' sorts earlier than '/' and cache_name_pos() finds
howto-index.sh, which is not under howto/ directory.  This
caused --others --directory to show it which was incorrect.

Check the directory name with the trailing slash, because having
an entry that has such as a prefix is what we are looking for.

Signed-off-by: Junio C Hamano <junkio@cox.net>