test: fix t7001 cp to use POSIX options

Since 11502468 and 04c1ee57 (both first appearing in v1.8.5), the
t7001-mv test has used "cp -a" to perform a copy in several of the
tests.

However, the "-a" option is not required for a POSIX cp utility and
some platforms' cp utilities do not support it.

The POSIX equivalent of -a is -R -P -p.

Change "cp -a" to "cp -R -P -p" so that the t7001-mv test works
on systems with a cp utility that only implements the POSIX
required set of options and not the "-a" option.

Signed-off-by: Kyle J. McKay <mackyle@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t7001-mv.sh

@@ -307,7 +307,7 @@ test_expect_success 'git mv moves a submodule with a .git directory and no .gitm
 	(
 		cd sub &&
 		rm -f .git &&
-		cp -a ../.git/modules/sub .git &&
+		cp -R -P -p ../.git/modules/sub .git &&
 		GIT_WORK_TREE=. git config --unset core.worktree
 	) &&
 	mkdir mod &&
@@ -330,7 +330,7 @@ test_expect_success 'git mv moves a submodule with a .git directory and .gitmodu
 	(
 		cd sub &&
 		rm -f .git &&
-		cp -a ../.git/modules/sub .git &&
+		cp -R -P -p ../.git/modules/sub .git &&
 		GIT_WORK_TREE=. git config --unset core.worktree
 	) &&
 	mkdir mod &&