Added_symbolic_ref function into porcelain module.

This implements `git symbolic-ref` command via Dulwich porcelain.

Signed-off-by: Jelmer Vernooij <jelmer@samba.org>
Python 2.4+ compat

Signed-off-by: Jelmer Vernooij <jelmer@samba.org>
Properly close pack files on exceptions.

Cherry-picked from max0d41 @ GitHub.

Signed-off-by: Jelmer Vernooij <jelmer@samba.org>
Add porcelain for diff-tree.
Add wrapper for commit-tree.
Add test for ConfigDict.iteritems.
Reimplement iteritems.
Add ConfigDict.itersections.
Update some copyright headers.
Move refs handling to a separate file.
Move read_info_refs from dulwich.client to dulwich.refs.
Move write_info_refs to dulwich.refs.
Move implementation of ObjectStore.get_graph_walker() to Repo.
Actually use Repo.get_parents.
Deprecate Repo.ref.
Fix test suite when run on systems without ~/.gitconfig.
Add configuration for tox.
Extend tox config to run without extensions.
Cope with functions not having func_name in pypy.
Fix docstring for StackedConfig.default_backends().

(Launchpad Bug 1252493)