t5520: test single-line files by git with test_cmp

t5520: test single-line files by git with test_cmp

In case an invocation of a git command fails within the command
substitution, the failure will be masked. Replace the command
substitution with a file-redirection and a call to test_cmp.

This change was done with the following GNU sed expressions:

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

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>