lib-submodule-update.sh: reduce use of subshell by using "git -C"

We write

    (cd <dir> && git <cmd>)

to avoid

    cd <dir> && git <cmd> && cd ..

that allows a breakage in one part of the test script to leave the
entire test process in an unexpected place.  Modern version of Git
allows us to do this more concisely with "git -C <dir> <cmd>".

Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/lib-submodule-update.sh

@@ -69,10 +69,7 @@ create_lib_submodule_repo () {
 
 		git checkout -b "replace_sub1_with_directory" "add_sub1" &&
 		git submodule update &&
-		(
-			cd sub1 &&
-			git checkout modifications
-		) &&
+		git -C sub1 checkout modifications &&
 		git rm --cached sub1 &&
 		rm sub1/.git* &&
 		git config -f .gitmodules --remove-section "submodule.sub1" &&