t6042: Ensure rename/rename conflicts leave index and workdir in sane state

t6042: Ensure rename/rename conflicts leave index and workdir in sane state

rename/rename conflicts, both with one file being renamed to two different
files and with two files being renamed to the same file, should leave the
index and the working copy in a sane state with appropriate conflict
recording, auxiliary files, etc.  Git seems to handle one of the two cases
alright, but has some problems with the two files being renamed to one
case.  Add tests for both cases.

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