Remote: make sure to take the contents of the id

Pass the contents of the buffer containing the git_oid to bytes() so
build a raw representation of its contents. Using bytes(ffi.buffer(...))
returns the representation of the buffer.

pygit2/remote.py

@@ -334,8 +334,8 @@ class Remote(object):
 
         try:
             s = maybe_string(refname)
-            a = Oid(raw=bytes(ffi.buffer(a)))
-            b = Oid(raw=bytes(ffi.buffer(b)))
+            a = Oid(raw=bytes(ffi.buffer(a)[:]))
+            b = Oid(raw=bytes(ffi.buffer(b)[:]))
 
             self.update_tips(s, a, b)
         except Exception as e: