t5520: replace test -{n,z} with test-lib functions
When wrapping a git command in a command substitution within another
command, we throw away the git command's exit code. In case the git
command fails, we would like to know about it rather than the failure
being silent. Extract git commands so that their exit codes are not
lost.
Instead of using `test -n` or `test -z`, replace them respectively with
invocations of test_file_not_empty() and test_must_be_empty() so that we
get better debugging information in the case of a failure.
Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>