dir: recurse into untracked dirs for ignored files

dir: recurse into untracked dirs for ignored files

We consider directories containing only untracked and ignored files to
be themselves untracked, which in the usual case means we don't have to
search these directories. This is problematic when we want to collect
ignored files with DIR_SHOW_IGNORED_TOO, though, so we teach
read_directory_recursive() to recurse into untracked directories to find
the ignored files they contain when DIR_SHOW_IGNORED_TOO is set. This
has the side effect of also collecting all untracked files in untracked
directories as well.

Signed-off-by: Samuel Lijin <sxlijin@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>