tests: use "test_cmp", not "diff", when verifying the result

In tests, call test_cmp rather than raw diff where possible (i.e. if
the output does not go to a pipe), to allow the use of, say, 'cmp'
when the default 'diff -u' is not compatible with a vendor diff.

When that is not possible, use $DIFF, as set in GIT-BUILD-OPTIONS.

Signed-off-by: Gary V. Vaughan <gary@thewrittenword.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t7005-editor.sh

@@ -38,7 +38,7 @@ test_expect_success setup '
 	test_commit "$msg" &&
 	echo "$msg" >expect &&
 	git show -s --format=%s > actual &&
-	diff actual expect
+	test_cmp actual expect
 
 '
 
@@ -85,7 +85,7 @@ do
 		git --exec-path=. commit --amend &&
 		git show -s --pretty=oneline |
 		sed -e "s/^[0-9a-f]* //" >actual &&
-		diff actual expect
+		test_cmp actual expect
 	'
 done
 
@@ -107,7 +107,7 @@ do
 		git --exec-path=. commit --amend &&
 		git show -s --pretty=oneline |
 		sed -e "s/^[0-9a-f]* //" >actual &&
-		diff actual expect
+		test_cmp actual expect
 	'
 done