t6042: add testcase covering rename/add/delete conflict type

t6042: add testcase covering rename/add/delete conflict type

If a file is renamed on one side of history, and the other side of history
both deletes the original file and adds a new unrelated file in the way of
the rename, then we have what I call a rename/add/delete conflict.  Add a
testcase covering this scenario.

Reported-by: Robert Dailey <rcdailey.lists@gmail.com>
Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>