allow 20 byte sha strings

Adjusted py_str_to_git_oid to allow 20 byte binary strings. This improves usability to other clients which already store their shas as binary strings, increasing overall performance as they won't have to convert them into hex beforehand, which would have to be converted back to binary form by libgit2 in turn.
Adjusted tests to accept binary shas.

pygit2.c

@@ -230,16 +230,22 @@ wrap_reference(git_reference * c_reference)
 
 static int
 py_str_to_git_oid(PyObject *py_str, git_oid *oid) {
-    char *hex;
+    const char *hex_or_bin;
     int err;
 
-    hex = PyString_AsString(py_str);
-    if (hex == NULL) {
+    hex_or_bin = PyString_AsString(py_str);
+    if (hex_or_bin == NULL) {
         Error_set_py_obj(GIT_ENOTOID, py_str);
         return 0;
     }
-
-    err = git_oid_fromstr(oid, hex);
+    
+    if (PyString_Size(py_str) == 20) {
+        git_oid_fromraw(oid, (const unsigned char*)hex_or_bin);
+        err = 0;
+    } else {
+        err = git_oid_fromstr(oid, hex_or_bin);
+    }
+    
     if (err < 0) {
         Error_set_py_obj(err, py_str);
         return 0;

test/test_repository.py

@@ -44,10 +44,11 @@ class RepositoryTest(utils.BareRepoTestCase):
 
     def test_read(self):
         self.assertRaises(TypeError, self.repo.read, 123)
-        self.assertRaises(ValueError, self.repo.read, A_BIN_SHA)
         self.assertRaisesWithArg(KeyError, '1' * 40, self.repo.read, '1' * 40)
 
+        ab = self.repo.read(A_BIN_SHA)
         a = self.repo.read(A_HEX_SHA)
+        self.assertEqual(ab, a)
         self.assertEqual((pygit2.GIT_OBJ_BLOB, 'a contents\n'), a)
 
         a2 = self.repo.read('7f129fd57e31e935c6d60a0c794efe4e6927664b')
@@ -55,13 +56,13 @@ class RepositoryTest(utils.BareRepoTestCase):
 
     def test_contains(self):
         self.assertRaises(TypeError, lambda: 123 in self.repo)
-        self.assertRaises(ValueError, lambda: A_BIN_SHA in self.repo)
+        self.assertTrue(A_BIN_SHA in self.repo)
         self.assertTrue(A_HEX_SHA in self.repo)
         self.assertFalse('a' * 40 in self.repo)
 
     def test_lookup_blob(self):
         self.assertRaises(TypeError, lambda: self.repo[123])
-        self.assertRaises(ValueError, lambda: self.repo[A_BIN_SHA])
+        self.assertEqual(self.repo[A_BIN_SHA].sha, A_HEX_SHA)
         a = self.repo[A_HEX_SHA]
         self.assertEqual('a contents\n', a.read_raw())
         self.assertEqual(A_HEX_SHA, a.sha)