test: new test-lib function to test for equality between files

We need to be able to test for the presence of a newline at the end of
output.  There's no good way to capture trailing newlines in bash, so
redirecting output to a file is the next best thing.  This new
function should be used when testing for output that is expected to
have trailing newlines.

The next commit will demonstrate the use of this.

test/test-lib.sh

@@ -441,6 +441,29 @@ test_expect_equal ()
     fi
 }
 
+test_expect_equal_file ()
+{
+	exec 1>&6 2>&7		# Restore stdout and stderr
+	inside_subtest=
+	test "$#" = 3 && { prereq=$1; shift; } || prereq=
+	test "$#" = 2 ||
+	error "bug in the test script: not 2 or 3 parameters to test_expect_equal"
+
+	output="$1"
+	expected="$2"
+	if ! test_skip "$@"
+	then
+		if diff -q "$expected" "$output" >/dev/null ; then
+			test_ok_ "$test_subtest_name"
+		else
+			testname=$this_test.$test_count
+			mv "$output" $testname.output
+			mv "$expected" $testname.expected
+			test_failure_ "$test_subtest_name" "$(diff -u $testname.expected $testname.output)"
+		fi
+    fi
+}
+
 test_expect_equal_failure ()
 {
 	exec 1>&6 2>&7		# Restore stdout and stderr