t/lib-pager: use sane_unset() to avoid breaking &&-chain

This test intentionally breaks the &&-chain following `unset` since it
doesn't know if `unset` will succeed or fail and doesn't want a local
`unset` failure to abort the test overall. We can do better by using
sane_unset() which can be linked into the &&-chain as usual.

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Reviewed-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/lib-pager.sh

@@ -3,7 +3,7 @@
 test_expect_success 'determine default pager' '
 	test_might_fail git config --unset core.pager &&
 	less=$(
-		unset PAGER GIT_PAGER;
+		sane_unset PAGER GIT_PAGER &&
 		git var GIT_PAGER
 	) &&
 	test -n "$less"