t5520: use test_cmp_rev where possible

t5520: use test_cmp_rev where possible

In case an invocation of `git rev-list` fails within the command
substitution, the failure will be masked. Remove the command
substitution and use test_cmp_rev() so that failures can be discovered.

This change was done with the following sed expressions:

	s/test "$(git rev-parse.* \([^)]*\))" = "$(git rev-parse \([^)]*\))"/test_cmp_rev \1 \2/
	s/test \([^ ]*\) = "$(git rev-parse.* \([^)]*\))"/test_cmp_rev \1 \2/
	s/test "$(git rev-parse.* \([^)]*\))" != "$(git rev-parse.* \([^)]*\))"/test_cmp_rev ! \1 \2/
	s/test \([^ ]*\) != "$(git rev-parse.* \([^)]*\))"/test_cmp_rev ! \1 \2/

Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>