t5520: replace $(cat ...) comparison with test_cmp

t5520: replace $(cat ...) comparison with test_cmp

We currently have many instances of `test <line> = $(cat <file>)` and
`test $(cat <file>) = <line>`.  In the case where this fails, it will be
difficult for a developer to debug since the output will be masked.
Replace these instances with invocations of test_cmp().

This change was done with the following GNU sed expressions:

	s/\(\s*\)test \([^=]*\)= "$(cat \([^)]*\))"/\1echo \2>expect \&\&\n\1test_cmp expect \3/
	s/\(\s*\)test "$(cat \([^)]*\))" = \([^&]*\)\( &&\)\?$/\1echo \3 >expect \&\&\n\1test_cmp expect \2\4/

A future patch will clean up situations where we have multiple duplicate
statements within a test case. This is done to keep this patch purely
mechanical.

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