test: use test_must_be_empty F instead of test_cmp empty F

test: use test_must_be_empty F instead of test_cmp empty F

Use test_must_be_empty instead of comparing it to an empty file.  That's
more efficient, as the function only needs built-in meta-data only check
in the usual case, and provides nicer debug output otherwise.

Helped-by: Denton Liu <redacted>
Signed-off-by: René Scharfe <redacted>
Signed-off-by: Junio C Hamano <redacted>