test: use test_must_be_empty F instead of test -z $(cat F)

Use test_must_be_empty instead of reading the file and comparing its
contents to an empty string.  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.

Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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