dir.c: make 'git-status --ignored' work within leading directories

dir.c: make 'git-status --ignored' work within leading directories

'git-status --ignored path/' doesn't list ignored files and directories
within 'path' if some component of 'path' is classified as untracked.

Disable the DIR_SHOW_OTHER_DIRECTORIES flag while traversing leading
directories. This prevents treat_leading_path() with DIR_SHOW_IGNORED flag
from aborting at the top level untracked directory.

As a side effect, this also eliminates a recursive directory scan per
leading directory level, as treat_directory() can no longer call
read_directory_recursive() when called from treat_leading_path().

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