Log Message:
0.18.3:

BUG FIXES
* Read config during porcelain operations that involve remotes.
* Fix headers of empty chunks in unified diffs.
* Properly follow redirects over HTTP.

IMPROVEMENTS
* Add ``dulwich.porcelain.update_head``.
* ``GitClient.fetch_pack`` now returns symrefs.
* The server now supports providing symrefs.
* Add ``dulwich.object_store.commit_tree_changes`` to incrementally
  commit changes to a tree structure.
* Add basic ``PackBasedObjectStore.repack`` method.