t3903: document a pair of directory/file bugs

t3903: document a pair of directory/file bugs

There are three tests here, because the second bug is documented with
two tests: a file -> directory change and a directory -> file change.
The reason for the two tests is just to verify that both are indeed
broken but that both will be fixed by the same simple change (which will
be provided in a subsequent patch).

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>