The last two tests here were always supposed to fail in the sense
that, according to code and documentation, mktree should read non-recursive
ls-tree output, but not recursive one, and therefore explicitely refuses
to deal with slashes.

Adjust the test (must_fail) so that it succeeds when mktree dies on
slashes.
Signed-off-by: NMichael J Gruber <git@drmicha.warpmail.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>