t5520: prevent field splitting in content comparisons

t5520: prevent field splitting in content comparisons

Many tests in t5520 used the following to test the contents of files:

	test `cat file` = expected

or

	test $(cat file) = expected

These 2 forms, however, will be affected by field splitting and,
depending on the value of $IFS, may be split into multiple arguments,
making the test fail in mysterious ways.

Replace the above 2 forms with:

	test "$(cat file)" = expected

as quoting the command substitution will prevent field splitting.

Signed-off-by: Paul Tan <pyokagan@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>