Fix parse_diff tests not calling correct function

The parse_diff function was moved out of Repository in the past, but
some tests weren't updated to reflect this change. So, the tests passed
because they expected any exception to be raised, but they weren't
actually calling the correct function (Diff.parse_diff).

Author: jorio

test/test_diff.py

@@ -334,11 +334,11 @@ def test_diff_parse(barerepo):
     deltas = list(diff.deltas)
     assert 2 == len(deltas)
 
-def test_parse_diff_null(barerepo):
-    with pytest.raises(Exception):
-        barerepo.parse_diff(None)
+def test_parse_diff_null():
+    with pytest.raises(TypeError):
+        pygit2.Diff.parse_diff(None)
 
-def test_parse_diff_bad(barerepo):
+def test_parse_diff_bad():
     diff = textwrap.dedent(
     """
     diff --git a/file1 b/file1
@@ -347,5 +347,5 @@ def test_parse_diff_bad(barerepo):
     @@ -1,1 +1,1 @@
     -Hi!
     """)
-    with pytest.raises(Exception):
-        barerepo.parse_diff(diff)
+    with pytest.raises(pygit2.GitError):
+        pygit2.Diff.parse_diff(diff)