dir.c: git-status --ignored: don't scan the work tree twice

dir.c: git-status --ignored: don't scan the work tree twice

'git-status --ignored' still scans the work tree twice to collect
untracked and ignored files, respectively.

fill_directory / read_directory already supports collecting untracked and
ignored files in a single directory scan. However, the DIR_COLLECT_IGNORED
flag to enable this has some git-add specific side-effects (e.g. it
doesn't recurse into ignored directories, so listing ignored files with
--untracked=all doesn't work).

The DIR_SHOW_IGNORED flag doesn't list untracked files and returns ignored
files in dir_struct.entries[] (instead of dir_struct.ignored[] as
DIR_COLLECT_IGNORED). DIR_SHOW_IGNORED is used all throughout git.

We don't want to break the existing API, so lets introduce a new flag
DIR_SHOW_IGNORED_TOO that lists untracked as well as ignored files similar
to DIR_COLLECT_FILES, but will recurse into sub-directories based on the
other flags as DIR_SHOW_IGNORED does.

In dir.c::read_directory_recursive, add ignored files to either
dir_struct.entries[] or dir_struct.ignored[] based on the flags. Also move
the DIR_COLLECT_IGNORED case here so that filling result lists is in a
common place.

In wt-status.c::wt_status_collect_untracked, use the new flag and read
results from dir_struct.ignored[]. Remove the extra fill_directory call.

builtin/check-ignore.c doesn't call fill_directory, setting the git-add
specific DIR_COLLECT_IGNORED flag has no effect here. Remove for clarity.

Update API documentation to reflect the changes.

Performance: with this patch, 'git-status --ignored' is typically as fast
as 'git-status'.

Signed-off-by: Karsten Blees <blees@dcon.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>