This fixes the bug that @nulltoken found (thank you!) where
if there were untracked directories alphabetically after the
last tracked item, the diff implementation would deref a NULL
pointer.

The fix involved the code which decides if it is necessary
to recurse into a directory in the working dir, so it was
easy to add a new option `GIT_STATUS_OPT_RECURSE_UNTRACKED_DIRS`
to control if the contents of untracked directories should be
included in status.