t5318: use 'test_cmp_bin' to compare commit-graph files

The commit-graph files are binary files, so they should not be
compared with 'test_cmp', because that might cause issues like
crashing[1] or infinite loop[2] on Windows, where 'test_cmp' is a
shell function to deal with random LF-CRLF conversions[3].

Use 'test_cmp_bin' instead.

1 - b93e6e3663 (t5000, t5003: do not use test_cmp to compare binary
    files, 2014-06-04)
2 - f9f3851b4d (t9300: use test_cmp_bin instead of test_cmp to compare
    binary files, 2014-09-12)
3 - 4d715ac05c (Windows: a test_cmp that is agnostic to random LF <>
    CRLF conversions, 2013-10-26)

Reviewed-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t5318-commit-graph.sh

@@ -254,9 +254,9 @@ test_expect_success 'check that gc computes commit-graph' '
 	git config gc.writeCommitGraph true &&
 	git gc &&
 	cp $objdir/info/commit-graph commit-graph-after-gc &&
-	! test_cmp commit-graph-before-gc commit-graph-after-gc &&
+	! test_cmp_bin commit-graph-before-gc commit-graph-after-gc &&
 	git commit-graph write --reachable &&
-	test_cmp commit-graph-after-gc $objdir/info/commit-graph
+	test_cmp_bin commit-graph-after-gc $objdir/info/commit-graph
 '
 
 # the verify tests below expect the commit-graph to contain