t5318: use 'test_must_be_empty'

A handful of tests in t5318 use 'test_line_count = 0 ...' to make sure
that some command does not write any output. While correct, it is more
idiomatic to use 'test_must_be_empty' instead. Switch the former
invocations to use the latter instead.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Acked-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t5318-commit-graph.sh

@@ -140,7 +140,7 @@ test_expect_success 'Add more commits' '
 test_expect_success 'commit-graph write progress off for redirected stderr' '
 	cd "$TRASH_DIRECTORY/full" &&
 	git commit-graph write 2>err &&
-	test_line_count = 0 err
+	test_must_be_empty err
 '
 
 test_expect_success 'commit-graph write force progress on for stderr' '
@@ -152,13 +152,13 @@ test_expect_success 'commit-graph write force progress on for stderr' '
 test_expect_success 'commit-graph write with the --no-progress option' '
 	cd "$TRASH_DIRECTORY/full" &&
 	git commit-graph write --no-progress 2>err &&
-	test_line_count = 0 err
+	test_must_be_empty err
 '
 
 test_expect_success 'commit-graph verify progress off for redirected stderr' '
 	cd "$TRASH_DIRECTORY/full" &&
 	git commit-graph verify 2>err &&
-	test_line_count = 0 err
+	test_must_be_empty err
 '
 
 test_expect_success 'commit-graph verify force progress on for stderr' '
@@ -170,7 +170,7 @@ test_expect_success 'commit-graph verify force progress on for stderr' '
 test_expect_success 'commit-graph verify with the --no-progress option' '
 	cd "$TRASH_DIRECTORY/full" &&
 	git commit-graph verify --no-progress 2>err &&
-	test_line_count = 0 err
+	test_must_be_empty err
 '
 
 # Current graph structure: