dir: update stale description of treat_directory()

dir: update stale description of treat_directory()

The documentation comment for treat_directory() was originally written
in 095952 (Teach directory traversal about subprojects, 2007-04-11)
which was before the 'struct dir_struct' split its bitfield of named
options into a 'flags' enum in 7c4c97c0 (Turn the flags in struct
dir_struct into a single variable, 2009-02-16). When those flags
changed, the comment became stale, since members like
'show_other_directories' transitioned into flags like
DIR_SHOW_OTHER_DIRECTORIES.

Update the comments for treat_directory() to use these flag names rather
than the old member names.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>