The DWIM magic in check_filename() doesn't just recognize
":/". It actually makes sure that the file it points to
exists. t4208 checks only the case where the path is
present, not the opposite. Since the next patches will be
touching this area, let's add a test to make sure it
continues working.
Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>