test: test_must_be_empty helper

There are quite a lot places where an output file is expected to be
empty, and we fail the test when it is not.  The output from running
the test script with -i -v can be helped if we showed the unexpected
contents at that point.

We could of course do

    >expected.empty && test_cmp expected.empty actual

but this is commmon enough to be done with a dedicated helper.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/test-lib-functions.sh

@@ -606,6 +606,18 @@ test_cmp() {
 	$GIT_TEST_CMP "$@"
 }
 
+# Check if the file expected to be empty is indeed empty, and barfs
+# otherwise.
+
+test_must_be_empty () {
+	if test -s "$1"
+	then
+		echo "'$1' is not empty, it contains:"
+		cat "$1"
+		return 1
+	fi
+}
+
 # Tests that its two parameters refer to the same revision
 test_cmp_rev () {
 	git rev-parse --verify "$1" >expect.rev &&