tests: use 'test_must_be_empty' instead of 'test_cmp <empty> <out>'

[git] / t / t4135 / damaged.diff

diff -pruN a/postimage.txt b/postimage.txt
--- a/postimage.txt     1969-12-31 18:00:00.000000000 -0600
+++ b/postimage.txt     2010-08-18 20:13:31.484002255 -0500
@@ -0,0 +1 @@
+postimage