Fix broken binary diff test

In a recent commit to libgit2, binary diffs were changed to have a
trailing empty line. This broke a test in test_diff because it compares
it directly against the string. I've added the extra line to the
expected output and the test now passes correctly.

test/test_diff.py

@@ -77,6 +77,7 @@ Pc${NM%FIhFs^kIy3n&7R
 
 literal 8
 Pc${NM&PdElPvrst3ey5{
+
 """
 
 DIFF_HEAD_TO_INDEX_EXPECTED = [