This fixes a regression introduced in 2e612731b5 (submodule: port
submodule subcommand 'deinit' from shell to C, 2018-01-15), when
handling pathspecs that do not exist gracefully. This restores the
historic behavior of reporting the pathspec as unknown and returning
instead of reporting a bug.
Reported-by: Peter Oberndorfer <kumbayo84@arcor.de> Signed-off-by: Stefan Beller <sbeller@google.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>