t6036: Add testcase for undetected conflict

t6036: Add testcase for undetected conflict

If merging two lines of development involves a rename/add conflict, and two
different people make such a merge but resolve it differently, and then
someone tries to merge the resulting two merges, then they should clearly
get a conflict due to the different resolutions from the previous
developers.  However, in some such cases the conflict would not be detected
and git would silently accept one of the two versions being merged as the
final merge resolution.

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