test: use test_must_be_empty F instead of test_cmp empty F

Use test_must_be_empty instead of comparing it to an empty file.  That's
more efficient, as the function only needs built-in meta-data only check
in the usual case, and provides nicer debug output otherwise.

Helped-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t1011-read-tree-sparse-checkout.sh

@@ -215,7 +215,6 @@ test_expect_success 'read-tree adds to worktree, dirty case' '
 '
 
 test_expect_success 'index removal and worktree narrowing at the same time' '
-	>empty &&
 	echo init.t >.git/info/sparse-checkout &&
 	echo sub/added >>.git/info/sparse-checkout &&
 	git checkout -f top &&
@@ -223,7 +222,7 @@ test_expect_success 'index removal and worktree narrowing at the same time' '
 	git checkout removed &&
 	git ls-files sub/added >result &&
 	test ! -f sub/added &&
-	test_cmp empty result
+	test_must_be_empty result
 '
 
 test_expect_success 'read-tree --reset removes outside worktree' '

t/t9010-svn-fe.sh

@@ -53,8 +53,6 @@ text_no_props () {
 	printf "%s\n" "$text"
 }
 
->empty
-
 test_expect_success 'empty dump' '
 	reinit_git &&
 	echo "SVN-fs-dump-format-version: 2" >input &&
@@ -208,7 +206,7 @@ test_expect_failure 'timestamp and empty file' '
 	test_cmp expect.date actual.date &&
 	test_cmp expect.files actual.files &&
 	git checkout HEAD empty-file &&
-	test_cmp empty file
+	test_must_be_empty file
 '
 
 test_expect_success 'directory with files' '