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

[git] / t / t4013 / diff.diff-tree_--stat_--compact-summary_initial_mode

$ git diff-tree --stat --compact-summary initial mode
 file0 (mode +x) | 0
 1 file changed, 0 insertions(+), 0 deletions(-)
$