t3404 & t7508: cd inside subshell instead of around

Fixed all places where it was a straightforward change from cd'ing into a
directory and back via "cd .." to a cd inside a subshell.

Found these places with "git grep -w "cd \.\.".

Signed-off-by: Jens Lehmann <Jens.Lehmann@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: jlehmann

t/t3404-rebase-interactive.sh

@@ -101,10 +101,10 @@ test_expect_success 'rebase -i with the exec command' '
 
 test_expect_success 'rebase -i with the exec command runs from tree root' '
 	git checkout master &&
-	mkdir subdir && cd subdir &&
+	mkdir subdir && (cd subdir &&
 	FAKE_LINES="1 exec_>touch-subdir" \
-		git rebase -i HEAD^ &&
-	cd .. &&
+		git rebase -i HEAD^
+	) &&
 	test_path_is_file touch-subdir &&
 	rm -fr subdir
 '

t/t7508-status.sh

@@ -808,7 +808,7 @@ test_expect_success POSIXPERM,SANITY 'status succeeds in a read-only repository'
 	(exit $status)
 '
 
-(cd sm && echo > bar && git add bar && git commit -q -m 'Add bar' && cd .. && git add sm)
+(cd sm && echo > bar && git add bar && git commit -q -m 'Add bar') && git add sm
 new_head=$(cd sm && git rev-parse --short=7 --verify HEAD)
 touch .gitmodules