t6036: avoid non-portable "cp -a"

b8cd1bb ("t6036, t6043: increase code coverage for file collision
handling", 2018-11-07) uses this GNU extension that is not available
in a POSIX complaint cp.  In this particular case, there is no need to
use the option, as it is just copying a single file to create another
file.

Signed-off-by: Carlo Marcelo Arenas Belón <carenas@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t6036-recursive-corner-cases.sh

@@ -1631,7 +1631,7 @@ test_expect_success 'check nested conflicts' '
 
 		# Compare m to expected contents
 		>empty &&
-		cp -a m_stage_2 expected_final_m &&
+		cp m_stage_2 expected_final_m &&
 		test_must_fail git merge-file --diff3 \
 			-L "HEAD"                     \
 			-L "merged common ancestors"  \